I'm the extreme minority in this, but I do not have a problem with Gamebryo. When FO3 was announced, I hadn't heard of Bethesda before, and so I bought Oblivion to see what they were capable of [in-house]. I was initially VERY impressed, and was eying their customized engine for suitability to a Fallout title. What I saw was that they already had many systems in place that could just be renamed. Gamebryo had been used for 3D/isometric style games before. [
Kohan2, and
Loki]
I was certainly looking forward to it; until the first screenshots began to surface. I had been anticipating a game built upon the foundations of Fallout 2, and not a plastered over version of Oblivion, (which by then, I had discovered to be shockingly shallow and repetitive; but wonderful to look at). I do believe that they could have produced a game whose in-game visuals matched the original game's rendered cutscenes... To have the the Harold in-game character model BE a fully formed version from the earlier game's dialog head, and to see the BOS (and remnant Enclave) strutting around in the original cutscene armors.
To have Fallout 2 style conversations where the entire polygon budget was spent on the lip-synced close up head. To talk to someone face to face ~Fallout 1&2 style, but with 3D graphics on par with the
Nvidia face demo of the day.
Needless to say, FO3 was a really harsh disappointment. Fallout was unusual [among other reasons] for its detailed talking heads. I cannot recall a single contemporary RPG that did similar ~outside of Wastland/Bard's Tale, and Gold Box series... All pre-1994 and not reactive or lipsynced. And yet Bethesda paid not the slightest attention to this unique and noteworthy feature of the IP.
**It occurs that KotOR 1&2 kind of did have this.
While it has quirks (and has frustrated me first hand when modding), my only real issue with Bethesda's Gamebryo, is how they chose to use it with FO3 & 4.
I just realized the thread is about what FO3 did well, rather than wrong... And to that I can only (repeat) that it did a fine job landscaping the world to look the part. What there is of the environment looks spot on.