Hallowed Ground is not complete invincibility though, alhough it's unclear exactly what sort of damage it no-sells and what it doesn't, normal physical attacks at least are (making it somewhat like the PLD's old 2 hour ability 'Invincible' in FFXI). In fact, it's shameful of me to admit it but it was for this reason for much of my career as a PLD here in FFXIV I actually rarely used it, so used to keeping it as a last resort in FFXI was I (and especially because in FFXI it was basically a giant 'hate magnet' - it generated the highest amount of enmity out of any ability in the game, save for WHM's 2 hour Benediction, not to mention it had a two-hour long cooldown), and I have to keep reminding myself "it is not the same here, it's not the same ability, you're fine to use it when you need to!"

Either way, I concur with the above about not being afraid to use Hallowed Ground liberally, especially when you get tons of trash mobs link which can quickly whittle your HP down. It can be a downright lifesaver at times.