So heres my general impression of grenades at the moment
Moderate damage, knockdown, very low action cost makes it a very powerful weapon. However you must be within 15hexes to attack, making the grenade suffer the problems HtH suffers from (just not nearly as hard to get in range, and the knockdown helps keep them in range)
I kind of wonder how broken are grenades. On paper they sound extremely broken, but i think the 15m range makes a huge difference (then again the fact that you can keep the person on the ground makes a big difference once you are in range). I would have to test them out personally, and have to fight against them more to know for sure, but it does seem to need some kind of rework.
Anyways i'll leave the question of whether or not they're broken up to others. For now here are a handful of ideas.
1.) I don't know if this is possible (or rather if the developers are willing to put in the work to code it this way) but it would be reasonable to have the ap cost of grenades depend on how far away the target is. If the target is extremely close then it only takes 4-5 ap (before perks/traits) At longer ranges the grenades take up more AP. Seems realistic, you have to throw harder to hit farther targets.
2.) Make greandes similar to rockets, only slightly more spammable, and shorter range to balance. Increase the current cost of ap enough so that you can only throw 2-3 grenades, but increase the damage to compensate (We don't want grenades useless, there needs to be some kind of use at least.)
3.) very simple increase to range, increase apcost by only 1-2