I find Zenith Bombards very annoying when the AI has about 100+ of them on one of its worlds, which is often once they are unlocked. Very *very* frustrating to try to remove them. High hp, powerful shots, tendency to kite, incredible effectiveness against 'hard' targets like golems or spirecraft and they effectively block use of transports without the use of a sacrificial lamb. I swear the AI unlocks them just to spite me.
On attack, they're usually less annoying though I don't like they can out-range missile turrets AND are immune to sniper shots; oftentimes I have to divert several bombers to deal with a single bombard that got translocated by a military command station and is now sitting pretty plinking away from the far side of the system at my turrets :\
When I unlock Bombards on the other hand, they're pretty handy sometimes, but the prohibitive cost, low shipcap, and the prevalence of radar dampening or immunity to antimatter on just about everything the AI has that I'd like to snipe...
I play with Simple shiptypes alot and Zenith Bombards are my vote for 'Most Frustrating Shiptype' in that class. -.-