I can see where you're coming from here and think it makes sense to be cautious. Of course all Ts in solo practice are not unethical, but for those that are, a solo practice is the ideal setting, especially if it's sexual. Like you said, it's the same with any abuser - the ideal way to take advantage of someone is be alone with them. In solo practice there are no colleagues, no supervision, no one to check in when something seems a little off. The only way unethical Ts in solo practice get caught is if clients report them, and many (if not most) clients who are abused do not file complaints. Someone who is going to hit on or sleep with patients might do it no matter where they are, but solo practice makes it much easier.