Merc healers do not get a Combat Res(the only one of the three that does not).
Merc healers are almost exclusively single target healers. We have Kolto Missile, which is a decently low CD aoe heal(ground targeted), that does about the same per target as a Healing Scan(without the talent HoT), maybe a little less. It really SOUNDS better than it actually is, to be honest.
We have no HoT ability at all, minus Kolto Shell, which is without a doubt the worst healing spell for all 3 classes. It scales worse and starts lower healing than anything else in the game. It currently is bugged and at least can trigger more than once every 3 seconds, but even if all the charges went off instantly, the healing done per global cooldown is pretty terrible. Compared to casting ANY of our other heals, it never is better to cast it. When I merc heal(very rarely) I usually toss it on someone who may take incidental damage every now and then, or put it on the tank before the pull. Refreshing during the fight is absolutely pointless unless you have a break where no healing is required. It is very close to just being a worthless talent point that you do not want to take.
Our single target healing ability is decent, but I have to wonder what they were doing while balancing between the 3 classes. Our instant cast heal is a fairly LONG cooldown(21 seconds is pretty damn long) and our Healing Scan is normally a 9 second CD with talent. We can then remove the 9 second CD for 10 seconds after we build 30 charges of support cell/cylinder. Comparing to the Sorc just between the 2 cast heals(Rapid Scan and Healing scan, versus the 2 Sorcerer equivalents that I forget the name of...), they do not have a cooldown on theirs and can basically spam it. It isn't like removing the CD always would make Merc healing OP, since chain casting it can run fill your heat pretty damn fast and the after HoT won't stack.
All in all, when I run my ops groups, I look for Sorc and Op healers instead of Mercs. This tends to be fairly easy since most people stopped healing on Mercs a long time ago on our server for the above reasons.
Sure, you can still heal, but of the 3, we are the worst. We should have the best single target healing output, but it seems some nerfs went overboard in beta and we never got retuned, as the Sorc can outheal us on single target pretty easily, especially when you throw in the Shield.