Gamebryo isn't an engine, it's a suite of modules to make an engine. Bethesda used the graphics module from NetImmerse (later renamed Gamebryo) for Morrowind, and either wrote or licensed other stuff to fit in the engine as they continued to use it.
The recurring problems are Bethesda-specific, but that's because of how Bethesda made and put together their engine.
Funny, considering how horribly their games perform. Even on beastly rigs, Fallout 4 can't maintain a consistent 60fps, and the graphical fidelity does not warrant that kind of performance. With weaker GPUs, performance is not as good as it should be either. And before you bring up the cost of AI, on a properly written engine that stuff should be spread across multiple cores, while the graphics are largely offloaded to the GPU, and so shouldn't affect each other that much.
I personally don't care about having a constant 60fps since I'm used to playing games on relatively weak PCs (where I'm lucky to get a consistent 30), but claiming they aim for "maximum performance range" is laughable.
Actually they are. As computers and consoles improve, there's a change in how to efficiently utilize them. 10 to 15 years ago, for instance, you generally worked with one core and used clever tricks to avoid processing what you didn't need to (and could rely on the increasing speed of processors to make up for the increase in complexity). But today, we've hit a wall with processor speed and have moved toward parallel processing, where rather than cleverly working out what to process and when, it's more about how to separate processing into independent tasks that work in parallel. Similarly, parallelization has brought processing speed to a point where the speed of memory access is an issue (whereas before, processors were slow enough to largely mask memory access speed, but that's not the case anymore with these newer paradigms), so even the way data is stored and accessed needs to be rethought. It gets to a point where building on older design philosophies doesn't work as it used to and it becomes a limiting factor.