On a very tl; dr note: it falls flat even though it has potential. Surely, this could be connected to how I feel towards the game itself (except for the music).
Sure, the "mixing rpg and some smt esque mechanics with bullet curtain shooter" is novel and fairly unique - I will give it that. However, the rpg system is simply too primitive for it to be fun once the "OOOH THEY SHOOT AND I HAVE TO DODGE!" thing wears out. Even the earlier DQs managed to be fun even with simple spells and enemy weaknesses, but ut does not have that as on the attacking side, you are only stuck with simple weapons usage (press on the bar on the correct time to hit) and most of the time different heart modes aren't even nearly as used outside of boss battles. To add insult to injury, apparently in prototypes of the game, there was a "spell" command. It would be more interesting if the fighting back happened more with you shooting bullets back on a certain way (I am aware of a heart color mode but come the fuck on).
A comparison in a way could be done to the Mario & Luigi series. While they play differently, they have the same base - turn based rpg battle system, you must dodge the enemy attacks, low stats. The difference between both, genres aside is simple: M & L have more interactions and ways to attack AND counterattack, thus making the game feel more "alive" and "fresh" and maintaining this for longer. Undertale doesn't get any of that. The different attacks are mostly restricted to a few modes, which is a pity.
Speaking of which, the game sorely lacks a boss battle only-mode. And no, po-mo "meta lol" does not excuse it.
Another basic error is the lack of randomization. The game is too "memorize this EXACT pattern" instead of "react to THIS pattern" which limits its replay value. Whether it is a good or bad thing depending on the story and how you view its "meta points" it is up to you.
Surely, one can say that the design "choices" were intentional, or that it works this way because of the creator's vision. But this is merely my opinion, and that is not touching upon the whole issues I have with the gameplay and story intertwining which I shall save for another post as it would get too long.