Second this! the snot rounds give them away so much and being healer means they are even easier to kill! kolto missile is also really awkward and often too slow to it's mark. Mercs are nowhere near as good as sorc or op healers ever and thats even when they tunnel for heal scores. and too right they need an escape that would actually be a fair nerf for the mercs to because when they are under pressure and decide to go down firing with disregard to heat they are sincerely DPS dump trucks some classes may prefer if they opted to disengage instead of leaving 10 stacks of fire DOTs.
For heals- Snot rounds should haste the CD on their bubble and buff it so if players see them they know its coming from something thats not going down easily or easily as they do.
For in general Auto-armour. this is a mechanic that saved crysis 3. if a merc maxes their heat then their bubble should auto deploy (as a safety measure to protect their alies from melt down
this is balanced because if a merc is careless they can deploy their shields prematurely so slow down with that rocket spam bub. but if they are dived on by say a ravage attack and are quick enough to dump out they're abilities the auto Armour will give them a fighting chance in CQC then if they have conserved their vent heat and are heal spec they cant be interrupted or if in pyro they can kite back.
Also a disengage should be an actual jet escape as ive said so many times target the area and fly to it but as above make it require the merc to be fully over heated and it can be a means of venting gas.
as for but hurt mercs You are dump trucks! mobile turrets or walking bombers either way you're job is to out DPS the enemy so get you're rotations in order, get you're heals where you can reach em and i can't stress this enough L2pick you're situations! sneak attacks, support fire,AOEs, stay with you're team in WZs and CC companions in PvPworlds you are ranged tacticians not ninjas