My gaming group during the 1990s mixed-&-matched modules & games on-the-fly. It was easy enough. Ignore the stats in the module. Grab stats for anything described out of the books for the game we’re actually playing. When in doubt, make something up that seems about right. Worked beautifully. Most of the AD&D modules I’ve played were played under GURPS, Rolemaster, Hârnmaster, Fantasy Hero, & al.
Likewise, I’ve read so many stories of people making great use of GURPS world-books though they never played GURPS.
Yet, for some reason, I’ve never liked the idea of generic modules. (i.e. Modules without stats.) My own experience, however, argues for them. It’s probably good for every game to have a few modules that give some better idea of how that game’s mechanics are meant to actually play out. Beyond that, however, why bother?
Maybe more importantly, we should just discredit the notion that modules need careful conversions. Just ignore the system specific stuff and handle things however fits the game you’re playing.