Some players posses inhuman sniping skill, which amounts to "being seen = death".
And I am still convinced that even under normal circumstances the merc team is stronger when there is a game with 4 players who are about the same level.
Spies usually become strong when:
1. The server is generally laagy, even more so if the current host plays as spy
2. When the skills are not balanced. The mercs have less room for mistake, so if one of them is not as good as the other, the spies can focus on the weaker one. If you turn that over and have an imbalanced spy team, then the better spy can still choose where he goes, he has the initiative, which makes it less of an impact for the spy team to be unbalanced.
I agree that mercs would win more than spies for now but I wouldn't say it's absolutely impossible to win at the same skill level. If you really want I can find a video of pros playing(and the spy had a crappy mate too that died in the first 4-5 min) and the spy actually managed to win.
Also adding with my own experience, the two I usually face with my usual mate have mountains of more experience than us, and I'm the most inexperienced out of them. We still manage win or almost win as spies nearly 40% of the time.
I would also like to add in some more things I think about the skill caps:
Spies: I think another reason it's so hard to play spies, is because it gets way harder as you go down the objectives. Take club house for example. A good merc team can camp up or down and prevent the spies from double teaming and even getting one terminal on one floor. This game the spies play with "Leave a terminal upstairs so the mercs don't all go to one level" is completely absurd logic. I had a mate leaving a terminal downstairs there once and that cost us the effen game. The mercs do not need to play along and can just camp downstairs without caring anything about the upstairs. The same thing could be said for museum. Two mercs can ignore the two bombs and camp down the cafe and disk drop off points. Again, I am completely aware that winning for spies is not impossible even under these circumstances, but it's damn well harder than it should be. There's also the issue with not hearing proxy mines if you don't have a headset. Having one can change the game, as you don't have to take your time in a room looking for proxy mines and can turn on thermal vision to assess as you run around the room. Once mercs know how to abuse the meta-game, it can become very problematic but not impossible for the spies. What I think should happen is that stuff around the map happens that put it more in favor of the spies as they get more objectives, like in Aqua. So it balances out the fact that mercs don't have to cover as much ground anymore.
Which also reminds me to talk about Aqua. I absolutely hate the fact that Greek gives the most changing factor to the spies on what is a nigh-impossible map without it. It gives single spies more access to tech for pressuring and a wider area to heal and run around and hide in while the other objectives like pirate or tank disk have a crappy effect that is easily mitigated by MT and EMF. This leads to mercs prioritizing Greek and can make it near impossible for spies to hack even a second. So if there are going to be map effects for objectives, they better all have equal effectiveness in hindering mercs or helping spies. I may even be stretching it by saying that Greek helps the spies a lot cause the tech bomb takes too long to blow up and the area is too small for a good merc team to miss the bomb.
Mercs: A good headset can also be very helpful for mercs too, further making it difficult for spies. You can hear a spy rolling in a vent or in the ceiling easily, and hear his footsteps from across the map in club house. You don't even need eax, I recently got a headset and am capable of hearing someone placing a cam or using a medkit from across the map. Hell, I can hear my partner reloading and scoping in and out from a substantial distance. XD. Another thing is the camnet. It is very powerful on maps like Polar Base as you can tell which fence side they're coming from or what vent they're going down. Even double teaming a merc can be very hard for two good players, as the merc can just keep his distance and guard the objective and shoot from a distance at the same time. Not to mention the jumping and the crappy grab detection the clients get.
Another thing that can make it harder for spies without good hearing equipment is random mines. The kind of mines you don't put in the usual places, which can greatly hinder spies as they don't expect a mine to be on that rail or wherever you put it. This may delay them significantly as they spend several precious seconds looking for mines. I for one, have this technique of mining the exit of an objective that is being done and THEN deciding to run in and stop the spy. The spy will think I had no time to plant, and when he runs away to the exit, KABOOM! Take note ladies and gentleman, this technique is great for a spy who is running back and forth between exhi and mono. You put a mine somewhere in that disk path as he's running around and once you go to one bomb, he'll leave to run for another and BOOM! he dies. XD