Well, bad code is bad code regardless of the paradigm. I've just had bad experiences rewriting some horrible OOP codebases and opted out to use as much functional style as C# allowed me to.

The main problem, as I see it, is that OOP encourages unnecessary abstractions and inheritance. These should be used as little as possible, because they typically increase complexity and make code harder to read and untangle. As an example, I've seen people define interfaces that don't essentially define anything.

Another problem is that OOP encourages mutable member variables. It's very annoying to try to understand code where class C inherits from class B that inherits from class C. Good luck debugging when the methods of C modify a variable declared in A in subtle ways.

As an idea OOP is very appealing. When I was younger, I would be thrilled to start designing a class hierarchy and interfaces when encountering a new programming challenge. Now I just try to think how to make things as simple and modular as possible.

Edit: of course bad functional code is also bad code. It's also very annoying to try to understand code where functions pass badly named functions around as parameters and use 10 function compositions in a sequence.