I've never had my Fundies card questioned before, and GUE cards state specifically what depths and gasses the diver is qualified for. If a dive op doesn't accept that, then I probably won't do business with them. But then again, for travel I tend to research where I'm going and who people are diving with and talk to the operation so they know I'm coming and we both have a clear picture of what is going on.
And yes, GUE/UTD/DIR is more than a certification IMO. It's about team and individual skills, and being consistent within the organizations as a whole. This weekend I was lucky to join a visiting DIR diver for two very nice dives and while the trip didn't work out as planned we were both able to adapt and have some real fun diving together. Sure beats having an unknown insta-buddy on a charter.
But really, you need to chat with local instructors (and their students) about your progression. Where the card comes from is secondary, IMO, to what you gain from the course in both knowledge and skills. You can't learn from an instructor you don't gel with, period. I've been lucky that most of my out-of-PADI instructors have been really good mentors and can really get a point across.
Another thing I like to find is an instructor that is doing the dives I want to do. If you want to dive caves, find an instructor that is actively fun-diving or exploring caves. I don't know how much you could learn about deep wreck diving from an instructor that does all of their dives on tropical reefs :/ . Same thing for an instructor for whom teaching is all they do, I like knowing that the knowledge I am gaining from them is being used outside of the classroom on real dives.
So find an instructor or instructors (do your research), talk to them about your diving goals, and if possible go diving with them before a class to see how you really gel in the water. And Have Fun!!
Peace,
Greg