Also, I have no idea how people think WHM is the "easy healer.
- No pets, cards, stances or class-designs to consider seeing as Lillies can be safely ignored - it's literally just you, the health bars and the enemies.
- A kit with minimal 'ramp up' to hit peak. Assize/Holy/DoTs/Stone isn't exactly hard to execute, made easier by multiple HotS to reduce healing requirement.
- Two strong oGCDs - if you're levelling up or spamming dungeons like OP suggests, these will be very handy for the inevitable tanks in gear 30-100il below the norm who think they're multi-pulling gods.
- No mana concerns once Thin Air and Lucid are on the table. Even AST's levelling up will have to consider holding Ewer on the road to max, whereas getting TA+Lucid on a WHM is GG to mana-issues indefinitely.

You can bang on about ease of pets, better damage overall and all those fancy details - but the majority of those regard min/maxing or later content, whereas the OP has asked about levelling up in dungeons. I can't speak for SCH, which I levelled up to 30 before I puked, but as a WHM main who recently levelled up AST (which was enjoyable, albeit frequent need to hold Ewers from time to time), WHM always was and likely always will be the smoothest ride, just not necessarily the strongest.