What's the difference between throwing multiple instant grenades and a stack of grenades going off at once? In terms of damage it's exactly the same. But aliens can react to every throw and you have to guess how many to throw with the "resilient grenades". So it's less unbalanced than instant grenades.
The only advantage of resilient grenades is for proxy grenades since it allows you to stack many. But that's also more realistic. In theory, they would all be triggered at once and would all blow up instead of one blowing up and taking out the other primed grenades.
Currently, there is nothing allowing unprimed explosives of any kind to explode when in an explosion. I'm sure it could be done (in the same way some objects explode upon being destroyed, like UFO Navigations), but it has not been.
It's a bit weird to use the absence of an "explosive explosives" feature to justify not stacking grenades when a stack of primed grenades is the same as one primed grenade triggering other unprimed ones (except you need the TUs to prime them all versus just throwing unprimed grenades).
You are right that it is a balancing issue. I would say vanilla EU grenades are at the weakest, instant grenades are the strongest, and TftD's "resilient grenades" are somewhere in the middle.
