Are you pretending to be retarded right now? The monsters in RE were the result of an engineered product that was specifically designed to change them. Most of the mutants in FO supposedly became that way just from radiation exposure. It's not even remotely comparable.
I didn't intend to post in a thread about retarded weeaboo slanty-eyed bullshit game, but then I did catch some Fallout reference and was triggered. So...
No, mutation in Fallout didn't come because of just radiation. It's because of FEV - a virus accidentally discovered while trying to find a cure for a New Plague, a world-wide pandemic. Virus, instead of just combating the virus in creatures, started to change them, and often in better ways. This gave way to the Supersoldier program, designed to create the perfect soldier, which are now known as supermutants.
Creatures, subjected to FEV start to change - rapidly, but not instantly. Changes are irreversible and end result greatly depends on the radiation level of the creature subjected to virus: the less radiated subject is the better is his state after transformation.
The mutation itself are not over the top at all. Ghouls have extremely slow metabolism, allowing them to live for centuries, but they do look like shit with almost no regeneration at all. Ghoulification requires the presence of FEV and high doses of radiation around - note that Necropolis was not to far from the The Glow, where FEV was originally researched and after the bombing, which caused the virus to leak. Ignore stupid Shittesda retcons with ghouls everywhere.
Supermutants have excessive muscle growth, high regeneration, high endurance, but are sterile (which was a fatal mistake in the eyes of the Master, but from the point of pre-war researchers was even necessary), and if original subject was highly irradiated the end result would be as retarded as average redditor. Perfect soldiers.
Centaurs are abominations created by Master by throwing several specimens in the FEV tanks.
Small injections of FEV into humans made psykers possible (yes, PSI is canon since Fallout 1, but PSI powers almost never mentioned in any Fallout game
Underrail)
First two Fallout games took place in California, where the most important FEV labs were located (Mariposa and The Glow), since why we see mutants roaming the desert.
So yeah, it's not a hard sci-fi at any rate, but it's believable and follows particular internal logic. It's just, you know, like Tim Cain and co actually cared about the believeability and coherency of Fallout universe, not just throwing some "cool" shit into the mix.