How does ERP work for mental compulsions?

Very well. I have a couple behavioral compulsions like counting and checking. But the bulk of my compulsions are mental. The key thing with ERP is not doing ANY type of compulsion during or just after the exposure. Doing a mental with render an exposure ineffective just as much as a physical one.

I hear imaginal exposures work quite well

Yep. I've done both. But doing a real life exposure works as well