So its ok to use Ultegra with XT cranks, for example?
The cups themselves are compatible. There are some other differences to watch out for in road vs mtb versions. The road versions have shorter threading since they only need to work on 68mm shells. MTB version is 68/73mm and brings more threading to compensate for what is lost when you put spacers on if you have to. Then of course there is the 70mm italian thread version for road cranks which for obvious reasons will not work on any MTB.