They've used Gamebryo (or a modified version of it) exclusively since 1999. Of course it's Gamebryo. That doesn't necessarily mean it's going to be buggy, though. The engine gets blamed when Bethesda's QA is really to blame. (Although I'm sure that QA is really tough for the kind of big, complex games they make.)
Gamebryo isn't the problem here, this is true.
It's Bethesda + Gamebryo where it goes to total shit.
I mean, Catherine was on Gamebryo, RIFT was on Gamebryo, EL FUCKING SHADDAI: ASCENSION OF THE METATRON was gamebryo.
If it was a different engine, Bethesda would still be lazy.