The real issue with Bethesda's additions to the franchise is that they're hollow gimmicks.
Compare Fallout 3 to FNV. Fallout 3 added (from what I'm reading on the wiki) bloatflies, yao guai, fire ants, and mirelurks. All they were really doing here was expanding the creature roster, which is fine I suppose, although the Nukalurk is beyond retarded and very typical of Bethesda hamfistedness. (They correctly realized that Nuka-Cola was a sort of minor symbol of the entire franchise, so they went crazy making Nuka-Cola into a series of gimmicks far beyond the item's real import.)
FNV kept all of those added creatures because there was no sense in not utilizing existing assets that were appropriate to the setting, although Obsidian used only a modified version of the Mirelurk King rechristened the Lakelurk and ditched the rest of the goofy crabman lineup because it was goofy and stupid and they knew that it was goofy and stupid.
FNV restored geckos, fire geckos, golden geckos, mantises, plants of dark soul, and giant rats to the franchise; added cazadors, which are cool as fuck and have extensive origin and lore tie-ins; added nightstalkers, which aren't as cool as cazadors, but also have interesting origin and lore tie-ins; made yao guai special by tying them into the location and lore of Zion (including a vision quest-like tribal encounter); made Super Mutants entertaining and unique again by restoring their sapience and personalities, reinstating the Nightkin, installing them at specific locations that made sense, and implemented the fucking greatest voice work of all time in any computer game, Blackrock Mountain Radio; and added bighorners, bark scorpions, and coyotes to fit and reinforce the desert theme.
The marked men, spore carriers, tunnelers, and even the ghost people (despite being featured in the greatest DLC of all time) are a bit more questionable as additions, but again, these do have extensive lore tie-ins and are far better than the extracurricular shit Bethesda added to Fallout 3 (aliens, redneck troglodytes, etc.).
I frequently harp on world-building as being the pinnacle of RPG design philosophy, and this is no different. Building creatures that make sense in the context of and truly belong within that world is part of good world-building. Bethesda basically just copy-pastes gimmicks into its games.