The fact that in this game things like procs, proper caster placement, mana management are not a meaningful thing paired with the slow GCD already is a difference, on top of that a lot of MMO healers have heals with different effects other "heal for X" or that at least have interactions in their kit while XIV lacks of that, being most of the interesting tools a time gated resource not tied to the player performance. That is without mentioning once again the homogenization they would have to undo, other mmos dont have a Sage/Scholar situation where they are almost the same job (and funnily enough, the difference is playstyle is because a dps tool, ED).
Then you should know how a rework that transforms the current XIV into that is not feasible in less than several years and how the healer gameplay is not just about how much hps you can dump, but how efficient you are to provide the most value to your party, be in the form of heals, buffs, CC or damage. You should know too how as you grow in skill you make more meaningful choices when healing that leave you with more time to interact with tools that are not healing and why those are really important and XIV fails on that front.and 2 Most that have healers, healers get to heal in 60+% of general instance content! In endgame content like high end raiding that goes way up.