First of all, let's not bash any groups here. Let's just discuss which comebacks you were a bit disappointed by. Which comebacks did you think didn't live up to the hype?
For me -
Red Velvet's Power Up + Really Bad Boy
BTS - Idol (I still like it, but I expected something else from it. If that makes sense.)
(G)I-dle - Senorita (I like the chorus, but that's about it. I'm gonna listen to it more to see if it'll grow on me.)
Gfriend - Sunrise (Don't kill me

)
Loona - Hi High (The song is okay, but I was a bit disappointed by it.)
Song Mino - Fiance (Even though I love Winner, I couldn't even get into the album....)
Which comebacks were you disappointed by?
Agreed on all of the GGs, though Hi High was technically a debut, not a CB, and Gfriend, well, I only recognize Fingertip. I don't listen to Winner so idk what I should have expected from Fiance but I didn't like it at all. Idol seemed like what you'd expect from BTS but I'm no expert on them either. Also:
Twice - DTNA - one of two kpop songs I dislike so much I have to mute it every time it comes on. Yes or Yes was initially a big disappointment but it's grown on me a bit since release.
BP - DDDD - really just more of the same from Teddy, it's getting old.
Mamamoo - Wind Flower - easily their weakest title track of last year, even if the rest of the album was very good. It feels like they weren't sure whether they wanted to go after the kpop or general public audiences with this, and it seems like they missed both.
EXID - I Love You - more of the same that we've been getting from EXID. The Shinsadong Tiger/LE thing feels like it's really run its course. Not a terrible song but nowhere near their best, and possibly even their weakest title since they had BESTie members.
Apink - %% - Yeah, hard to follow up on one of the best songs of 2018, and it's not terrible at all, but I was really hoping for something even better than I'm So Sick.
Itzy - Dolla Dolla (debut, not CB) - Unlike a lot of people here, the more I listen to it, the less I like it, and I didn't like it that much to begin with. All other things aside, just not what you expect from a JYP debut song.
AOA - Bingle Bangle - AOA without Brave Bros. is a recipe for disaster.
Chungha - Gotta Go - the song itself is fine but I don't particularly like how much of her voice we get, especially in the chorus. She's the kind of vocalist who benefits from getting drowned out by other production elements.
Momoland - Baam - same damn song as Bboom Bboom, don't believe me, play both vids at the same time. Sure, there's a benefit to copying the same song if it's good, which both of their 2018 titles were, but obviously it didn't work out for them the second time around and they might as well have tried something else instead.
CLC - No - just wasn't into this. I'm glad they got wins that they should have gotten years ago for better songs, and that they finally got a CB at all, but this wasn't up to the standard of their previous singles in this style.
Gugudan - Not That Type - The Boots was lowkey one of the best gg songs of 2018, and Semina was a great summer track, so this was a bit of a letdown. Decent song and another display of their versatility, but not up to their other two songs last year.
CocoSori - Mi Amor - cute enough I guess, but I had come to expect them to fill the void left by less serious groups like Orange Caramel and this just wasn't it. Too bad we won't have a chance to find out if they would have gone back to broccoli and j-rock.