In my opinion, sparks’s weakness is currently the same weakness proxy and aura also have, which is probably higher with those two : they might have a too low health pool to be reliable against good tracking/good explosive skills players. And their speeds do not compensate enough currently.
Their main weapons are not helping either. Blishlok, even though deadly in good hands, has a poor dps output and hochfir is also a medium tier smg, below kek and crotzni definitely. Kira has meanwhile an easy way to deal with this : dreiss/br16/stark plus laser beam which are strong enough on long range to make her decent.
Sparks’s revivr is also quite efficient on long range, which make Sparks a better choice than Aura as a second medic.
Currently, I feel like there’s a bit too much 1 HTK on those nimble mercs. Fletcher, Nader, Skyhammer (indoor beacon) have to be considered as real strong threats that are straight light stompers. As well as semi-auto snipers which 1 HTK on head, or bolt action snipers which will put you between 1 hp and 16 hp on a bodyshot (Aura and sparks will be left at 1 hp against a fel-ix, proxy at 16 hp against moa, no damage drop off considered).
I will also mention that light mercs suffer more from the anti-lag. I won’t discuss this too much, but my idea is that they have a larger health pool fraction unusable in close quarter before going into cover. This strongly contradicts aura and proxy fast hit & run natures. [And the effects from anti-lag are also more noticeable (kills behind objects) as they run faster.]
After all this, I came to the conclusion that, even though I love playing light mercs on pub games, Aura and Proxy are nowhere near of being wise choices in skill balanced mm games or pugs.