Nebula's not uncounterable. Throwing 16k metal of Nebula at a spot with 4k metal of mobile AA nearby to deal 500 metal of damage before spending 1500 energy repairing and recharging is not a good deal. And it's quite feasible on the scale of a large Nebula fleet for the opponents to put together a mobile AA group that can wipe out all the drones in one salvo and then blow up a Nebula through all the shields in the next.
The main design problem I see with Nebula is Krow shock but worse. It's one thing to demand a team scrape together 2k metal of AA plus flex to confront a surprise Krow, and another to demand them to come up with 4k metal of long ranged AA with no more notice to confront a Nebula.
A secondary design issue, only a problem when combined with its long range, is that its ability to fly backwards makes it a whole lot more agile than its turn rate would indicate. Instead of spending ten seconds turning around as an airborne pinata, it can spend a couple of seconds decelerating to its drone release point and then speed away faster than most AA can follow, all while altitude and retreat advantage make it risky for the AA to press too close while it's turning.
There's a specific interaction between Nebulas and Artemis where a group of Nebulas finds it profitable to just tank a few missiles on their shields and fall back to sponge shields off a friendly Aegis or two. Given the cost difference I don't think it's a real problem.
