DVD Studio Pro vs. DVD specifications / norms

Hello!

I have a few questions about DVD Studio pro in general:
I've been doing several projects with the software, and so far, it always worked well.
One of my Projects will be replicated soon.

To do a replication, I got in contact with professional DVD-Authoring Studios.
I was told that the studios don't use DVD Studio Pro, because it wouldn't work 100% according to the DVD specifications.
I have heard these statements from different Authoring Studios, and now, I really want to know what is all about.

What exactly are these problems with the specifications, if there are any ?
And how can I avoid them ?

Thanks a lot!

Paul