Merc not losing there bite is fine. However, i do feel they could still do that on a slightly more level field when it comes to vision. The player is never allowed the combination of durability(armor,hp,resist), vision(NV,thermal) and camo. The player can never match them for camo and NV, but can match them for thermal, and durability with end game tech.
The only reason i'm really enjoying any consistent success against them is by abusing the AI. They are top tier in every meaningful stat all at once. I would prefer if they had some meaningful weakness like other factions. Even stargods have there relative frailty in counterpoint to invisibility and phenomenal offense. Raiders are durable but weakly armed. Church has nasty terror/elite units and better weapons but frail.