What are the limitations on using onion address relays and amethyst pointing to orbot socks v5 port ? Isn't the same as having tor native at amethyst ? 

 Default for orbot is one entering node and one exit node. So, all your accounts are seen together.  

 But i think this is only for exit nodes .. if you use onion address relay you don't exit at all, you stay inside onion net.. isn't? 

 Yep, no exit.