I see I'm not the only one willing to point out non Bethesda games that were not plagued by immense bugginess thanks for the signal boost.So here's the difference between Bethesda's engine and engines like Unity and Unreal: Bethesda isn't trying to sell their engine on the side as a core part of their business model. Companies like Crytek and Epic Games have a massive incentive to hype their engines because it not only sells he games they make—which double as sales pitches—but also drives other companies to invest in the engine itself creating a feedback loop. Gamebryo was actually licensed and used by other devs, and looking at the list of games made using it, the problem doesn't seem to be exclusively the engine. Games like Civ IV and Catherine didn't seem to suffer from using it.
(heck in the case of Civ 4 their use of Gamebryo was so straight forward and tweaked so little of it , that linux gamers could launch it with wine fairly rapidly after release ... That's how solid that version of Civ was )