They're fine and can even be great, but not really how I would usually choose to spend my time in-game. It's nice that it's there for the people who like it, but please don't gate the true end or best weapon or whatever behind catching leviathan for which you need the best rod for which you need to learn the card combat system for which you need to have played the card game the whole game, and so on.
I understand that integrating rewards from different mechanics is a cool idea, but unless the game is stellar to begin with I don't usually have the interest to delve super deeply into every mini-game, you know?
That said, NieR was something really special and I'm still annoyed at the dude who gave it a bad review because of the fishing.