I've been using Cover liberally during dungeons runs just so I get a feel on how to actually use the ability efficiently. I've counted 2 successful saves over many runs and this is because apparently a lot of players don't like it when you use cover and run away to break the tether. I think breaking tether isn't necessarily bad but I don't think there are any other skills in the game that has another player dictate the usage of your abilities. There were also times where Cover would've been perfect, but that 50 Oath Gauge cost makes using it even more difficult.

It's a situational ability that punishes you hard for using it at the wrong time. I think increasing the tether range (and creating a visual indicator for when the tether is about to break) and lowering the cost is a good direction to start. It could have an effect similar to Synastry where a heal on one target is also a heal for the chosen ally but that might be stepping into Astro's turf.