I generally prefer FP style for most things as well, but sometimes FP can be taken too far.

Trying to understand type signatures that are 3 lines long with nested class types/generics and getting your code to fit into some kind of monad transformer pipeline can get confusing very fast.

For problem domains where you have a lot of state that is shared all over the place (like trying to implement a vector editing program, electronic schematic editor/simulator, or game development), I find OOP models easier to work with.

Monads can be used when you have shared context, but then your code gets constrained and a bit unnatural; you have to worry about Kleisli composition, endofunctors, and plumbing things into and out of the state monad. Yuck.

FP is a tool. OOP is a tool. Know their strengths and weaknesses and when to use each. Yes, you can work entirely with a single paradigm but IMO you're handicapping yourself by doing so.