WHM's strengths not only do not get designed around but often actively designed against with stuff like damage downs and one shots. The three mitigation healers all have content designed for their strengths because mitigation is king. (the so called regen/pure healer split is outdated, the proper categories now are Mitigation Healers and White mage).

WHM/AST is a really bad combo and the numbers show it. When my giga casual static cleared some of the fights there were single digit amount of logs for that healer combo.