Don't get me wrong, Kirby: Planet Robobot is a great game, and a great Kirby game (Return to Dream Land is still my favorite though). It still uses the same general formula as KRtDL as well, but while looking through my pros and cons of this game, I found that various ones were about the same thing. I had some things that I both liked and didn't like at the same time, hence "bittersweet." While I'll probably still talk about some of the things I exclusively enjoyed, I'll mostly be going with some of the topics that had a bit of both.
To start off, Copy Abilities. I've never had a Copy Ability I explicitly hated (not even Sleep, mind you), and that's still the case here, although some obviously have more love than others. The game takes a lot of Copy Abilities from Triple Deluxe, which is fine, but I'm a bit bothered how they're all almost direct ports with seemingly no changes whatsoever (exceptions are Spark and Ice, which have the lightning and ice ball attacks return respectively). Even with Spark getting some moves back, they're not to the same calibre as Return to Dream Land, with less range. However, I'm going to let this slide for now. Previously I'd made a blog post noting the buff and nerfs to Copy Abilities in Triple Deluxe, and one of the changes I noted was range (particularly with Fighter). However, I'm brought to believe these decreases were due to Kirby's size compared to the screen; he takes up more space compared to Return to Dream Land or KSSU, so range changed to compensate. As for the veteran Copy Abilities Mirror and Jet, I was definitely glad they were brought back, and with some new moves to boot (mostly Mirror). For new Abilities, I became attracted to ESP for its sheer power, Poison for comical reasons involving repeatedly breathing smog (mimicking deep breathing), although I had less attachment to Doctor until I found out how powerful it's aerial dash attack is when holding the button (Pharmacy).
Then there's Smash Bros. and UFO. These are probably the abilities that got the most excitement, but at a cost (figuratively and literally). Smash Bros. (which I'll just refer to as Smash from here on) is a powerful Copy Ability, but is quite rare, being found only in certain secret areas or in the Ability Testing Room after beating the game, or using the Smash series Kirby amiibo. I like how they referenced the Hammer Flip as it was implemented in SSB4, but it's not without problems. Aside from not being as powerful as Hammer's, it requires charging and can't be done quickly in comparison to its previous incarnations. The smash kick also has 3 phases instead of 2. This can be beneficial because more distance and power, but it takes more time in order to do such. Smash also has Giant Swing and Ultra Giant Swing, in addition to the Twinkle Star attack. This feels rather superfluous, and a bit of a step back, as Ultra Giant Swing does more damage than Giant Swing, in which the past versions didn't have the both of these. However, I do appreciate the unique Master Hand statue that Smash can transform into. As for UFO, it cannot be obtained until after 100%-ing the game, or by having the Kirby series Kirby amiibo. While it does have some new moves (and can go down soft platforms), it feels slower than usual. I like the tractor beam attack in concept, but I wish it had more to it than just sucking an enemy in (perhaps act like Saucer in Epic Yarn). Also, furthermore, neither of these abilities can be accessed in the Arenas, so if you want to play them using each ability (such as myself), you need the amiibo (fortunately I had Smash Kirby, but still).
Sound effects are still good, but I feel like a few were changed, and not for the better. Archer's Magic Arrow's SFX doesn't have as much punch to it, Smash's Twinkle Star has been changed, Security Force's sword swipes sound like Kirby's instead of Meta General's, Galacta Knight's True Arena exit doesn't quite fit in my opinion (although the erratic flight part beforehand has the right sounds), and maybe a few others I'm forgetting.
Damage numbers. I'm happy they brought damage numbers back for measuring's sake, but it came with its downsides too. The data was only obtainable from Security Force, a mid-boss (less health than bosses and that half damage bit that makes measurements more difficult), and it seems to be revealing that damage has decreased in many moves. Hammer Flip was a common standard at 80, and Spark/Plasma Wave at 40, but now they've been altered to 76 (excluding extra hits) and 38 respectively. The damage alteration may not be a major change, but the fact that they were changed like that at all feels unnecessary (some kind of make sense, though).
Holo-Defense API was an interesting concept for a boss, but it also felt lacking. You're fighting 4previous bosses with watered-down health. Magic arrows can one-shot some of them I think. This makes the boss pretty non-threatening (although if you decide to do this without any abilities, it can be a decent challenge). I appreciate bringing back Ice Dragon, but it'd be nice to see him in the flesh again instead of being a sort of clone every time now. Another thing about Holo-Defense API is it can be a bit of a background boss. It bothers me when a boss spends a long time in the background with hardly a way to combat it, only avoid attacks until it comes back. While this boss doesn't explicitly do that, when you're trying to get that last hit as it flies erratically (especially if you're going for a good time in the Arena/True Arena/Meta Nightmare Returns), it gets quite annoying every time you manage to miss it; sure, it eventually stops and goes to the center, but it can still be quite a pain.
There's a new mechanic added into the game: momentum canceling. I've noted some things like this since Return to Dream Land, like how Fire's Fireball Spin would always roll if performed from a dash jump regardless of distance from the ground, the amount of Beams Blasts that can be performed in a single jump, etc. In this game, using an attack after a jump will omit any vertical momentum. I noticed this when using Beam; I tried to perform a Wave Beam in midair, but as soon as I started charging, Kirby would just fall to the ground and his upward momentum would vanish. This can be beneficial to some abilities (such as performing aerial moves lower to the ground, particularly with Ice for me), but harmful to others (like as I just stated with Beam).
Now, onto the positives. Stickers were a nice addition to the game, being the replacement of keychains. It also helps that the location of Rare Stickers is pointed out in the Sticker Room, whereas Rare Keychains didn't have this luxury, so you'd have to resort to recording them yourself, or looking them up on the wiki, assuming somebody had already done such.
The Arena and True Arena I feel have been improved by having the bosses in the same order every time. I'm a person who enjoys consistency and doesn't particularly like randomness.
Extra stages surprised me with the music. As Triple Deluxe's only used music from KRtDL (aside from the Dream Spring theme), I was expecting something similar with this game. Instead, I was invited to songs from various past games, like Air Ride, Star Stacker, and many more. I also appreciate the introduction to Casino stages, a theme I was fond of (because of Sonic 2). I also find it comical when playing as Meta Knight and doing a sword slice at a giant billiard ball, sending it flying in the opposite direction. The ability to play at Meta Knight again was also a nice addition.
This game has lore, and I can appreciate. Return to Dream Land seems to have been the main start to the lore (aside from maybe the Dark Matter Trilogy), and this game does continue it, with references to Dream Land 2, 64, Return to Dream Land, Triple Deluxe, Super Star, and so forth. There's also the ask-a-thon, which can answer some burning questions, although miiverse does make me feel iffy because I feel some people say/do some stupid stuff, but that's nothing I or Nintendo can control (aside from not allowing comments, if that's possible).
Lastly, the additional sub-games. Kirby 3D Rumble made reference to one of the beta stages of KRtDL, and I'd like to see something like this as a full game one day. Team Kirby Clash was also a fun experience, adding medieval costumes to the Copy Abilities and altering them slightly. I'm also fairly sure that an amount of people would like a Kirby RPG, in which only this and Kirby Quest have managed to do, albeit as just mini-games. There may also be a possibility of "2.0" versions of the games, similarly to Kirby Fighters Deluxe and Dedede's Drum Dash Deluxe, but there hasn't been any official talk of them, and if they will exist, I may feel a bit iffy due to not being the most fond of paid DLC.
That's what I have to say about Robobot. If you managed to read this all, you either have too much time on your hands, or you've been to the procrastination station (just like me).