My favorite aspects of Undertide were easily the unique art direction and the story. I've never seen a game that looks quite like this! I also enjoyed the music, but the way it was delivered was a bit odd. I LOVED the way many of the buildings looked, along with the distinct color palette
When the music DID play, I liked it a lot, however it seemed entirely random when and where it played. There were tense moments I was expecting to hear some kind of creepy ambience, but instead there was silence. Later on some creepy ambience played in a random unrelated spot, unprompted, so I wasn't sure if it was a bug or intentional
The story was fun, and really ramped up fast towards the end. I might have accidentally triggered some story beats out of order, because I was very confused when the stage director was having me recreate a scene, but I had no idea what was going on but it kept forcing me to start over
Mechanically, the game felt very 'fragile', as in if I didn't move in exactly the intended path, I could break the whole game. I was relieved to see there was a fully unlocked chapter select to compensate for this. The collision detection was rough, because I often found myself getting stuck on decoration, clipping through walls, and I even fell out of the world entirely at the beginning
Lastly, it wasn't very clear what I could and could not interact with. Many of the NPCs blended together, so I didn't know who I could and could not talk to. There were also doorways that I thought were decoration, but ended up being essential to story progression. Some kind of effect to indicate interactivity would have smoothed things out quite a bit
Overall though, this was an unexpectedly nice game. My criticisms are mostly 'polish' kinds of things, so I'm looking forward to checking out your future projects!