1 Its fandom is one of the biggest cancers of the Internet, almost rivaling Sonic, FNAF and MLP at times
It made porn involving the Amalgamates (and Alphys).
I don't think you can get much more degenerate than that. At least it was done ironically, I guess...
This isn't an "actual" problem with the game itself.
It's the worst, tied with Five Nights At Freddy's.
2 Its storyline is just about the most overrated thing to ever be written for a video game
3 Its popularity has overshadowed equally fantastic achievements of indie game design way too much
4 The game is almost mind-numbingly easy
5 Its characters are somewhat mediocrely developed at times
6 Its graphics, while not terrible by any means, are very lazily designed and clearly made to cash in on an overused style
7 Its emotional tone is inconsistent
8 It tries way too hard with its humor
9 The Genocide Run contains not one, but two absolutely outrageous difficulty spikes
10 It is quite possibly the most pretentious game ever created and tries way too hard to reinforce its message that killing is bad
The Contenders
11 Mettaton doesn't appear enough in it
12 Alphys should have had her own boss battle
13 The fact that Alphys X Amalgamates is a real thing that actually exists
Why. Why do you people do this.
14 Not being able to skip the dating segments
15 Alphys won't stop calling you on her phone during the Hotland segment
True, it's kind of annoying
16 Its fans seemingly have no idea when to stop overpraising it
17 Temmie should have had her own boss battle
Temmie is female? I thought she was a guy this whole time.
Technically, she did.
18 Too short
19 Its popularity has inevitably lead to the formation of a massive legion of salty, unappreciative, disrespectful haters
20 The game is a Rule 34 magnet to the point where it even has its own R34 subreddit called Undertail
The game isn't at fault, though. That's just gross people.
Ugh, yes. The porn. It's everywhere.
21 Most of the Amalgamates' overworld sprites are wildly inaccurate to what they actually look like in battle
Yeah I never understood that. It's a very minor flaw, and doesn't take anything away from the general experience, but it mildly off-putting
For example: Lemon Bread looks like a guy wearing a hoodie.
That's actually true. They look too different.
22 Asgore sucks at naming things
Home? New Home? He probably named Hotland too. And his son's name is just his name + his wife's name.
23 Sluggish fast-travel system
24 Its dialogue is so melodramatic and over-the-top that it almost feels like a bad fanfiction at times
25 Bosses dying in literally one hit on the Genocide Run