[Xabre]

Honestly, it's not all that powerful. Each Piranha is worth 2 drones. You have to leave the board at the end of the movement phase, so you're not firing with the pirhanas, and then you're opening for the reserve roll to get back on the board....

If you just want to spam drones everywhere, go with Drone-Net XV1-0. better drones.

 

[Xabre]

If a unit moves off the board they enter ongoing reserves and show up again the next turn. As to the drone dumping over an and over again, does the formation stat they get all their drones back when they come back onto the board?

It doesn't guarantee that they show up again the next turn. They simply enter ongoing reserves, which means you have to roll for them to come back onto the board.

And yes, the rules say any damage is repaired, and any seekers and drones are replaced.

RAW, it does not say that the drones have to be off the table killed or anything like that.

Rearm and Refuel:If all of the surviving models from a unit in this Formation are within 6" of a table edge at the end of their Movement phase, the unit can enter Ongoing Reserves. When it returns to play, it does so at full strength with any damage repaired and Drones and seeker missiles replaced

 

[Xabre]

Fair enough.

I guess the real question becomes... why?

Like I said, I'd probably much rather actually run them from the start. You pay 40 points for 24 points worth of gun drones, and yes, by turn 2 you'd have 48 points worth... but it's just gun drones, it's still only BS2, and it's still only carbines for 18" shots. I'd rather just run Drone Net, get BS3 drones with access to skyfire and interceptor and stuff like that, have a huge mess of them on the board to start with, and not worry about them being anchored to the edge of the board.