One keynote for White Mage, regardless how squishy your tanks are, Holy is generally superior to healing as long as the stun resistance hasn't fully ramped up so you can stun enemies for a collective amount of 7 seconds each - basically like a 7 second hallowed ground for every pull (assuming mobs can be stunned). After that it depends on how your tank mitigates (if at all *shudders*) and how you rotate your healing cooldowns in coordination with eventual Healing SPELL usage.

That said, going into Shadowbringers as a tank player, I feel like the equivalent to the damage intake I have now is equivalent to me tanking in Stormblood without my tank stance on, on top of a handful of mitigations (like blocking, parrying or Dark Mind) having dropped in overall power while boosting other parts of their kit (mostly short-duration cooldowns). All in all, my experience in Shadowbringers is like me playing a DPS-stanced tank for the majority of content.

But generally, disregarding metric & cooldown changes going into Shadowbringers, the main two issues is either the tank is literally running minimum item level armor and thus his defense + HP is squish, or he is plainly not mitigating, OR both.