There's a continuum here between "reusing old assets in new and interesting ways that disguises the old asset or makes it feel new some other way" and "obviously reusing old assets with little effort (like palette swaps) to save time and money on labor costs."

I don't mind the former. There are some enemy designs where I had no idea they shared a base animation rig until I saw it pointed out to me because the characters built over the rig looked so unique that it didn't even occur to me.

But Dawntrail had A LOT of enemy mobs and especially dungeon bosses that were very obvious reskins of old monsters. When I start to notice to the point it pulls me out of the game, where the illusion breaks and the immersion bubble pops, that's when I feel it's started to cross a line and be an issue. "Wow, it's that one tentacle monster model from Void Arc again but now it's a computer program tentacle monster that looks really silly and out of place...oooh..."/s

A lot of Eureka monsters were reskins, but the reskins still felt artistically unique to me, I guess? It's a fuzzy line, I will admit. For an example in Dawntrail that I think works, Honey B Lovely is a reskinned Shiva in her spear stance, but she doesn't look, behave, or have battle effects much like Shiva, so I don't really mind. But when it's like "that one enemy mob you've seen literally everywhere, but GREEN this time!" it's like, man, come on. Try a little bit harder, maybe? Reusing assets is fine (it'd be stupid not to reuse some assets), just put some effort to make them look nice.