Recently, I've had a rare need to check interwiki links. I've discovered that many times, the universal login credentials are not working.
For example, after login to english, the french link seems to accept the credentials (most of the time), while the german link *never* accepts the credentials.
Moreover, there is still the problem with secure, in that following interwiki links doesn't stay with secure, so have to re-login for each site. (Also, secure is slower than ever, and edits sometimes log the IP instead of the user name.)