It's not powerful at all. Its cooldown is way too long and when you consider this the amount restored is a drop in the bucket. Adding insult to injury is that it has to be one of your ships, meaning you're losing a ship and in most cases the resources to make said ship. Sure it has synergy with the third ability and some of the other Advent Rebel tech like the ship Resurrection (not that I've ever seen it work) but I still think its a stupid idea. I'm disappointed with the Advent Rebels as a whole. Oh well.
I love them, once resurrection/clone enemy frigate thing are working properly I'll probably start to focus on them. Guardian is but one of the many tactics to use with that ability, and I wouldn't be surprised if one I started using gets nerfed eventually.