its more then that really which is sad:
1)the lore of the game so not relevant to their kits its like you are told of an amazing mages and your charecter has the kit of a derp from another planet cause he ignored common logic and went the derpy way.
examples: ast varied card buffs, diurnal/nocturnal sects , sch strategic side of the kit, sch 2 different fairies, whm elemental magic and sylphie lesson about using all aspects other then just cure I spam.
2) baited free cure/benefic that are not that helpful and much more punishing cause you don't heal so well cause of it.
3) gcd healing punishes you for using them and hard, whm suffers from mp issues so use cure III (the "best" heal) and suffer cause it drains your mp so much you can hit rock bottom from the first cast and before you ask/think no problem he probably can handle it then wrong. whm is the most mp heavy among the healers and guess how "good" his mp regen with thin air? the answer is go back in time to shb thin air, cause now skill lives to its name now (meaning it worth nothing).
4) your leveling experience is skill drought for 10-15 whole levels and ur job gimmicks for example whm is at 76 meaning from 1-76 you dont have your full gauge and gauge skills and sch have a gauge for 1 use only which u get at level 70,and 2 skills that nuke half your healings skills at your hotbar at 60 and 80 cause reasons.
5) others can heal, tanks can heal ,dps can heals even the trust npc can heal themselvs(and if they are healers they can do both and have more options then the actual healers), meaning a healer doesn't even need to be there ppl treat themselves instead, combine with damage so low u can just use hp potions and be fine as well(u don't need to use but it exists) .
the only thing ppl can recommend whm for is ppl that don't want to play a healer and want to go afk.
i wont be surprise if the reason trusts exist is cause they planning to send the healer roles to oblivion and simply pumping everyone with changes filled with despair to get rid of the last fools that have any hope things get better.
*sorry, had to vent out and give a much more detailed description to your post.