Look, I know that Undertale is a complete package where every moment feels more purposeful and it's less disjointed but can I just say
I kinda love that Deltarune is all over the place and that each chapter has a distinct vibe. I love how it does UI manipulation and 4th wall breaks more randomly instead of only during key plot moments. I love how it crams a bunch of ideas together regardless of whether it would mesh well as a bigger picture. I love that it's a lot more esoteric in it's messaging that leaves more room for personal interpretation. I love that the animation quality randomly becomes really really good sometimes you might even feel it's out of place.
When I play Undertale, its a consistently GOOD game overall. Deltarune has some DOGSHIT moments, but plenty more GREAT moments.
It's a passion project with love pouring from all sides, from a creator that wants this to be his everything. I just like this kinda thing more than an objectively well produced game, it feels more alive.