Difficult clients. Yup, that's the thing that's always stopping me from having my studio rented out. At least, I'm the only difficult client my studio has to deal with. And I care about the equipment, so that ain't too bad.
Most of the time, I don't mind having to deal with 'difficult engineers', for as long as I'm sure they're helping me get the sound I want. It's really the output that matters to me.
And when I talked about damage to equipment, I didn't mean normal wear and tear. For example, if you installed new drum heads, and after the recording session the head is dented, that ain't normal. The guy has poor technique, and I'll keep my ribbon mics farthest from him. Or if I let somebody use my synth, and afterwards my synth is missing a volume knob. That ain't normal. Not to legit musicians.