Alright lets recap:
Assualt - lv1combo, lv2, lv3
medic - lv3 combo, Lv4combo
Warrior - Lv3(total gg reavers)
assasain - Lv3
Mech - Lv1, Lv3(His exp soon dwarfs summoner's), Lv4(at temple)
Spec Ops - Lv4
LM - Lv2, Lv4(see mech)
DM - can still help out by cursing heros that might be guarding it.
Mutant - Lv3, Lv4(see mech)
Volt - Lv2? can lure for a while into cannons
Archer Lv1pumped, Lv4(if you got the hp, or at temple)
and lets not forget cloaked and invinc. attacks cause them to just give up on their destructive path to your temple.
Yet somehow reavers are totally broken and unstoppable(after like 5 nerfs, when they have actually been very beatable since the second nerf)
Assault: Yes.
Medic: Can counter if he has high damage allies. Requires spending L4 to kill reavers... which is not going to put you ahead.
Warrior: Most rarely get L3, if it neutrals reavers that's another counter.
Assassin: Yes. (During the night, anyway.)
Mech: You're going to have to cast L1 4-5 times to take down each reaver. Have fun with that. L3 is viable, if he's gotten it and a ton of mana, which most don't. If he's sitting at temple trying to counter reavers, you've already lost. L4 won't do much either, manawise.
Spec Ops: With L4? Lol.
LM: Has to be careful not to die while casting, can obviously be a good counter given the enormous damage. Again, if you're defending reavers at temple, you've already lost unless you have something doing equal damage or destroying the reaver spawns.
DM: Useless.
Mutant: I'd be surprised if you could get two reavers with L4; definitely not mana cost effective. L3 is powerful if you get close enough, provided you don't die running into the reaver cluster.
Volt: I don't think L2 lures spawns. Not sure if L3 neutrals reavers.
Archer: Needs to get too close to do damage with mutas well. HP required, which makes mutas less effective against a unit that already has a ton of armor.
Summoner: Useless.
So that's 2-3/12 classes that can effectively counter, and 3-4 that can counter if you sit in the heal and rely on reavers for all of your experience. They give a lot, but it's not that much. This isn't even taking into account that any smart team that gets reavers is going to be right next to them blasting back at you or stunning you when you come within reaver range, and the reavers will have pounded through your front door in no time.
I don't feel a team should have to pick their heroes thinking "if they get reavers, one of us has to be able to counter them." The hard counters are rarely played units, except for the Warrior. In 2v2, or after a teammate has been eliminated from a 3v3, a lot of games can be ended simply with reavers if a team knows this. I still feel they are overpowered, and though there are equally or more serious issues in the game, I wanted to make that clear.
None.