While Rick can give you extensive answers, I'll give it a shot.

Hypnosis, in and of itself, is safe. Generally, you keep control and won't do anything you wouldn't in normal life.

Yes, when in the realm of hypnotherapy, it can have drawbacks if you do not see someone who is certified. Since you have memories that you haven't dealt with (your brain, that is) if someone who is an amateur connects with something that is traumatic, and doesn't know how to handle it, you can have big adjustments and reactions. There's no need for that.

However, self hypnosis is always safe because your brain (you) won't allow yourself to dig up anything you aren't ready for.

Few hypnotic suggestions are held beyond a few minutes after the session. If you want the suggestion to help you, though, then it can have lasting effect for the positive. Again, no suggestion that is beyond what you would allow when fully awake will take hold.

tc