Both SCH and WHM are viable for every single content in the game.
With that being said, WHM viability depends on the group itself a lot more compared to SCH, solely due to MP issue.
If people keep getting hit left and right, dying up and down, the strain on a WHM's MP is heavier compared to a SCH.
Thus with a SCH in the team, you are allowed to F up more, but with a WHM, you are allowed less .
I can see this being the main reason people prefer SCH to WHM for 4 man content.
For 8 man, having 2 SCH means MP issue is most likely non-existent, and I am thankful this thread has not caught up yet, so my WHM is still viable .