I don't actually think that there is any problem with a massive barrage of flares, but there comes a point where too many flares might become a hindrance to the game rather than a boon. I mean, imagine a squad or platoon of infiltrators all behind the walls of a base and all of them launch their flares at once before the big rush by their team. All the defenders would be blind and helpless outdoors just in time to be siege'd on by lightnings, main battle tanks, and liberators. But that opens the issue of the blinding effect, that is probably the only part of light flares to become an exploit and a con.
On the note of IR flares, it would be hazardous to the soldiers of their Empire all wearing night vision goggles or implants, and no one likes that kind of trolling. So this is why I might suggest a maximum light amount from cumulative flares being as bright as daylight. This way, a platoon can fire off a bunch of flares at all angles around a base and create "daylight" for a few short minutes so that the tanks and snipers can line up quick shots. And the IR flares? Imagine a night skirmish happening between two bases, no lights, moderate cover, and a couple guys fire off a couple IR flares. All of a sudden the entire battlefield is lit up providing more illumination than normal night vision can offer. I rather like it
And I am fully expecting them to be at least as wieldy as normal grenades (there still will be grenade launchers, right?). Don't get me wrong, I think it would also be a good idea to create a flare gun to take up a pistol slot so that anyone without access to grenade launchers could still use them, but I'm just expecting them to be a grenade for ammunition purposes.
And if they were sticky, that would offer a different kind of mechanic that, while amusing, would defeat most of the purpose of firing flares in the first place. When you fire a flare close to the ground or stick it to a wall, you're limiting the maximum amount of light emitted to certain areas. This does, however, have a pro to it, that being that you can fire it to the ceiling underneath cover that would be otherwise shaded from a launched flare, still illuminating the area. But that's where darklight or normal night vision shines best anyway. Perhaps we could compromise and make different modes of light for flashlights?! Press the toggle once for normal light, press it again for IR?