Advantage to Taric's ult: It only helps allies. Enemies aren't protected.
Disadvantages: the windup and very obvious visual indicator telling enemies EXACTLY how long they have to kill you for free, otherwise wait literally 2 seconds and kill you then. It will do absolutely nothing to any team fight including an assassin OR ADC.
Bard ult advantages: Range. You don't have to be in a stupid location to use it. hitting enemies and towers... yea, they're invulnerable... but it can be used to chase, escape, or tower dive. Taric's ult's wind up means it CAN'T be used in any situation where you're chasing or being chased... which, realistically, is MOST team fights because you're either engaging or being engaged upon. It's not really all that common for both teams to find an agreed upon turf to stand their ground that long on (except when you're turtling because the other team's at a huge advantage and you're cowering behind your towers... and an ult that only becomes functional when you're in that position is a little sad.