I've also felt that way. Can't forget mysty step. The two things oath of devotion has are sanctuary and sacred weapon.
Sanctuary makes it so that nobody can attack the target. That's /sometimes/ safer than healing, but your ally now can't attack without losing it.
Sacred weapon adds the spellcasting modifier to the weapon and I believe chance to hit? That +2-3 helps great weapon master's -5 penalty.
Edit: it also evades physical resistances as a magic weapon. This is strong against enemies that have both types of physical resistance.
Holy Rebuke is terrible. Turn the Unholy is situational.
If used optimally, oath of devotion can have higher damage (through improved hit chance). With bless, it's pretty much like the -5 penalty doesn't exist. That's a permanent +10 to base damage oath of the ancients needs to sacrifice hit chance for.
Some hardcore D&D players might know more than me, but half the spells are pretty good. Of course, ancients gets moonbeam for guaranteed ranged damage and can heal better.
