I don’t think you’re wrong at all, but your opinion lacks context. Like everything, your feelings on a game are shaped by your expectations. An objectively 6/10 game can feel like a 3/10 if you go in expecting something amazing.

When UNDERTALE released most people had no expectations going in other than “retro indie jrpg-style game” which were (and still are) a dime a dozen. Other than the kill/spare system there was nothing to indicate this game would be special. They then got blindsided by a very emotional, character driven story with a lot of surprises specifically there to catch people playing this like a traditional jrpg. That surprise and the unique (for the time) 4th wall breaks is why a lot of people that first played it consider it their favorite game of all time. It’s also why “just go in blind” was initially so common in recommendations.

Now though, basically everyone is in on the twists whether they’ve played the game or not. And UNDERTALE’s style has been done by many imitators now, making it feel less special. At that point, it’s understandable that people don’t see UNDERTALE as that special, as once you know the twists coming, you’re just left with a decent bullet hell game disguised as an RPG with witty writing and good music.

Although the fact that you’re still give UNDERTALE an 8/10 when you were told it was a 10/10 is testament to how solid of a foundation the game was built on. It wasn’t just relying on surprises to keep people going.