Complete Monster: Abaddon continues to live up to this, having entire planets destroyed in his wars.
Crack Is Cheaper: Good luck tracking a set down without shilling out a few hundred bucks. Doesn't help that it's not officially produced anymore.
Cult Classic: Is still quite a popular game, the miniatures are relatively easy to put together and paint, and the rules are quite simple, and the overall theme of a space naval combat tabletop game is relatively unique.
The Necrons. While most of the factions were reasonably well-balanced, the Necrons were obscenely overpowered in several different ways. They get the only armour saves in the game, their light cruisers are easily capable of going toe-to-toe with battleships of other factions, and their inertialess drive gives them incredible speed. Like their 40k equivalents of the day, the Necrons were overpowered by design, offset by a rule that granted more victory points than a ship's value to an opponent that managed to kill one, but it wasn't enough to compensate for the raw power the Necrons brought to the table.
Fighters/bombers/assault boats (and any ship that could carry them), at least initially. In the first iteration of the game's rules there were no limits on how many of these things you could have; as long as you had models for them, your fleet could basically continually pump out fighters out of their hangers every turn, which made escorts semi-worthless and turned the game into a contest of who could build up the largest cloud of fighters the fastest. A homebrew rule - which was later canonized in a later ruleset - made it so that the maximum number of starfighters on the board could not exceed the total number of launch bays in its parent fleet in order to address this issue.