I think it is FINE to state up front you don't want to work with suicidal clients, but to take them on and work with them for months or years and THEN decide it is too much is completely unethical unless you terminate in an ethical manner. It's WRONG to just dump clients who are suicidal and "difficult" with no warning.

Terminating if someone isn't getting better is okay too, but it is the manner in which it is done that can be SO SO damaging and traumatic. A decent therapist won't just dump a client. WRONG on so many levels.