A lot of times the issue with Diurnal Astro paired with a White Mage is they will think they are another White Mage. So they will play like one when the job and skills aren't exactly that, and sometimes both thinking they are main healers. So Diurnal with White Mage many times ends up in overheal. I can assure most say something like they just "prefer the stance". So they don't care that WHM regen skills have more potency per tick and the skill regen is longer, and that is where the double regen argument ends. Astro AB is not exactly regen, it is a front-loaded heal then regen ticks. So common scenarios WHM that had regen a GCD or more quicker than the AST and they are now at full HP, so now their late AB is already an overheal. Things like that add-up.

Then the obvious factor of not having mitigation for fights that can need it like Seph Ex, Thordan Ex, A4S. If those primal LB bars/stacks build up too high, you have no shield mitigation to protect against those, and effectively killed most if not all the party. Now the Astro has no one to blame but themselves. Of course those primals (not A4S) have been solo healed without mitigation, but it's not always a perfect world either and the job of a healer to keep everyone alive. The wrong stance can be part of that factor.