I ran one game of Afermath! and played in maybe a half-dozen campaigns throughout the 80s and early 90s. It is one of those 70-early 90s trad game that has a lot of complexity that at the time I really dug ('It's so realistic!' or 'It just makes sense that ____'), but now I take with a heaping pinch of 'does this really benefit the play experience?' salt. The level of detail varies from 'useful... if something is important to you,' to 'complexity that serves little to no purpose. Examples of the earlier include: each six-second combat round broken up into segments, so you know where everyone is when a given action occurs and the 'everyone moves and then freezes while someone else acts' effects are reduced to incredibly granular bits; reach and zone of control; rules for stopping a charge or other continuing-momentum issues, and the like. Examples of the later include things like: starting age (which influences how skilled your character is at the expense of attributes, not unlike Traveller) is determined by first determining a 0-5 age band, and then rolling <fixed # based on that band> + 2D
5; guns do damage based on bullet used (honestly relatively realistic, the bullets have a listed Bullet Damage Group (BDG), and then the damage is Roundup(BDG/10)D10 + Round(BDG/10) -- so a BDG of 19 does 2d10+2, a BDG of 21 does 3d10+2, and 25 does 3d10+3
(any of this seeming better in any way than saying .44 ammo does 3d10+2 and leave it at that?).
Overall, I don't think Aftermath! is in any way a bad game (although I think an updated version ought definitely go through the rules and parse between complexity with a purpose and otherwise). It is, however, a game with a level of detail I don't see playing again (unless things radically change with my playstyle.
I also have to admit that Aftermath was the only post apocalypse game I'd ever purchased (even though I never played). I never had Gamma World, nor any other game of that genre.
I think Gamma World to Aftermath was definitely like Shadowrun to Cyberpunk -- a similar thing but also retaining some of the fantasy elements of the D&D with which most people started TTRPGs. Purists might scoff, but many gamers enjoyed the extra toggles that provides (different species to play as, magic as treasure or alternate build choice, etc.). Possibly a reason why Aftermath! wasn't more popular, but I'd say the complexity
I liked the hit locations in Aftermath! since armor can be piecemeal and over lapping can stack. Example: Combat boots and shin guard could give a higher armor value for the shin.
That is a great example of complexity that serves a point (just not one I find important). This I can see keeping for a theoretical reboot/clone of the game, whereas other stuff I would clip immediately.