Are they?

I mean the tooltip says they're an upgrade of Frags, with smaller blast radius but a lot more damage. But with my current skills it takes me two Frags to kill a Rumbler, but five or six Disruption nades. Weird.

yes, this is an original bug (disruption grenades not getting any bonuses), fixed in ADaoB.

Hmm, I would swear I installed all the mods recommended in the sticky thread, including the one you've mentioned... probably messed up the order, I think I installed ADaoB first, and then everything else on top of it.

Oh well, too late to cry now, for the next playthrough I'll try to read the install guides more carefully :)

yeah, there has been an issue with the GOG release, ADaoB and ss2tool 4.9 or older (fixed in 5.0), guess you are one of the few (un)lucky users affected.

repatching with 5.0 should fix it, but that would also mean starting a new game.