Well the gamebryo engine does have some very interesting traits that make it amazing, but I agree its clunky and old. It has a massive level of persistence (shoot an arrow, fast travel to location in the distance, get hit by arrow you shot) and some amazing individual NPC character tracking and abilities.I suppose I differentiate between mods for the sake of creativity/user content and needing mods just to make the game playable/stable/de-consolized. My opinion is that Bethesda is pretty lazy/mediocre in relying on the community to mod away poor UI and bugs and such.
Edit - they are also pretty so-so in the writing department, and I have never found either their graphics or combat to be that good. And that scrappy old engine of theirs, Gamebyro or whatever it was called. Rickety. Really, about the only thing I think they do well is build a large varied open world.
Its not a slouch of an engine, but it is long in the tooth and bethesda games have been slowly going down hill narrative wise starting with Oblivion.
But I will say, there is no other game that gives me the same feeling of living that waste lander fantasy as Fallout 3/4, which is why I forgive the plot. I only follow it by accident, I'm here to get out of my vault and wander the wastes as I progress from a scrub in underwear into a Thanos like monster. Pretty much the same for TES games.
The radiation kills you ending in FO3 made me laugh, as I was largely a rad resistant monster also in power armor, but its not the first time a game just ignores the character you have in favor of a narrative.