Well, CORE 2e, the PHB and even the first few softback source books, is manageable. It is still AD&D with all its warts though. The combat system is a little less ambiguous about certain things, but it is still pretty muddy as to exactly what the process is for being 'in melee'. Some of the optional rules and subsystems are unworkable, but assuming you have a bit of received lore you can easily run it without TOO much trouble. OTOH oddly the exploration rules are much LESS developed than in 1e. Dungeon exploration turn structure is basically AWOL from 2e.
I think the fundamental problem is that there's no really solid substructure to build on. There isn't any formalized mechanism such as feats and powers, or even a single consistent casting system, so when things got added they were just all over the place. This is a place where 4e, particularly, excels. Roles and power sources nail down the thematics of classes, and their niche, and then its pretty obvious how 'builds' can be incorporated, feats are pretty consistent and have lots of examples, etc. 2e didn't have to get messy, but the discipline of constructing a core, and then building your baseline options using the same rules that are used to build new stuff later on, made 4e much less likely to 'go crazy'. 3.x is a bit in between, and I think overall 5e does pretty well in this department. 1e would have had 2e's problems, but for whatever reason TSR eschewed that sort of extension back then (wiser heads?).