One problem I have run into recently is that for a complex form you are
not necessarily able to tell which step is the last. E.g. after you submit
the login form and MediaWiki verifies your credentials, depending on your
user settings you might or might not be presented with a two-factor
challenge; so submitting the user name and password might or might not be
the last step of the form. (Arguably login should not be constructive in
the first place, but it is now. In any case, similar problems could be
present with the user registration form, which does create something.)
Personally, I agree with Bartosz that having four button types (five or
more if we include silent buttons) just makes the interface confusing.