Yes disc priest and holy paladins are strong, however I've raided with plenty of terrible holy paladins, and don't get me started on most of the resto shaman players I've raided with, Absolute garbage. It's pretty rare that I come across a good resto shaman, I'm my 4 yrs or so of raiding I've only raided with one decent resto shaman.
Any healing class is worth taking to a raid if they know wtf they're doing
Resto Shaman is the worst class for healing in 10m, sure there are one or two fights where they might shine, but overall a twink druid/monk with much lower gear has a higher output. This results mostly out of the misery that people need to stay in healing rain/chain heal range so that I shaman can heal efficienty, but as every encounter has like 3-5 abilities (fire, void zones, jumping debuffs etc) the raid will need to stand split up most of the time.
Holy Paladin and Disc Priest are the most solid choices, and then just pick a 3rd healer (druid, monk, resto). For 2 heal fights, Holy Paladin is almost a must, with the second healer filling the niches a paladin cant (so for example absorp heal with disc or AoE/movement heal with druid/monk)