FM is inherently more difficult to understand than subtractive, even with a very good interface. It's the double-edged sword of FM - you can create very complex behaviours from a small number of simple operators, which makes FM both uniquely powerful and uniquely difficult to think about intuitively.

I can model subtractive and additive patches quite accurately in my head. That's not a brag, it's just not very difficult if you understand the basic principles and you've put in a bit of practice. FM still takes me by surprise after decades of trying to master it.