Undertale is one of my favorite pieces of media, but I don't expect the average gamer to agree. It hits home with a lot of people that are "into video games" for what they bring to the table in the sense of subverting expectations. As someone who grew up playing JRPGs (Final Fantasy 3(6) and Secret of Mana hold a huge part of my childhood heart), and someone who has continued to follow the video game scene closely, I understand all the video game tropes we've seen so far. Also, I think it helped that I'm "into internet nerd culture" enough to understand all the anime tropes (even though I don't really watch much at all). Undertale just hit a lot of the checkboxes I didn't even know I wanted it to hit on my way through it. I don't do game development but I watch a lot of videos on the regular about game dev and I also enjoy watching videos about the movie industry and how to tell stories. I find that stuff very interesting, and there is something very interesting to me about how the developer tells the story in Undertale as well as lets you decide how you want to play the game.
The gameplay though...
The gameplay is...um....not very compelling. It's a different spin on the RPG genre and it changes it just enough through the game where you're often saying "oh, I see how that works now" and then the game moves onto something a little different. There were a few times where I felt a little bored with a few of the battles. Once I hit that "I get it" moment, I don't gleam much else. With that said, the game
generally moves onto the next leg of the adventure pretty quickly.
It really isn't for everyone, but for the people it's tailored for, it hits just right. I hope this post is somewhat helpful to at least one person at some point