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.