I've gotten a lot of reports of not being able to log into various instances with Tusky. It's MEANT to be able to log into any instance, so it's not by design. It also seems for a given instance some users will be able to log in and others can't so it's not even necessarily an instance-dependent bug. Right now I know of one SSL-handshake related bug but I don't know for sure the login issues are all a single bug yet.