The focus is the range not the lifespans. It could give sight too far behind the line.
Brits have borderline flares. SU has cheap flares and really easy to use (Damn! they already have the mortar im position to use it too!). USF has rifle flare as you stated.
OKM has flare -> OP! need nerf! #Alliefanboysunite!
How many times must i say this: Balance its not in each unit nor ability, its in each faction/commander overall design.
If you team up with someone to use flares+skillbomb. Thats not OP, you just wasted so many options and resources to one specific "combo" and a real expensive one.
want rifle flare on OKM?? OST jaeger have them, they are rarely used. you want that? Already Pzfusiliers have them too, not original change at all. Still want flares nerfed?? NERF THEM ALL.
What I said earlier, many flares gives you sight for so much time, even mortars get accurate shots, i mean thats cheessy and sort of "maphack". Make them last a few seconds, buildings light up with it will remain "visible" but not targetable, sort of like what tracking does.
Edit: Damn i want a revamp on all flares now! No more Fog of War revealing! Thats cheaty! Now they only do reveal invisible units inside their range. Same lifespan. Costs to be considered.