That's actually where your problem is. Most comments about DPSing as healer come from a SCHs perspective. WHM can do it too, but exponentially more dependent on the party members being decent or better at avoiding/mitigating damage (standing out of the bad, tank cd's, etc).
The other thing is just how the roll of the die works when it comes to matchmaking. Some folks just get the bottom of the barrel, others get the creme of the crop. SCH can manage with a larger margin of those players, whereas WHM isn't as fortunate on the fulltime DPS opportunities.
Edit: Also keep in mind that the comments are on Expert dungeons as SCH (mostly). Outside of the lowest level dungeons, healing in leveling ones are usually hectic. Healing Aurum Vale, for example, is still a nightmare given the type of party members one would usually get. I've had a couple beautiful AV runs from time to time before, but far more terrible ones that couldn't get past the first room.
