I'll see what I can come up with for screen shots. That said; this is the setup. Maybe you can see for yourself.
Mods: Double Supply/Cap, Enhanved AI, Expanded research, max starbase upgrades, more tactical slots, E4X mod
Game setup: Vast map, Vasari with other Vasari as ally on team 1 (Basically Vasari vs All).
Maxed out other enemies that are evenly split between that factions. Player types set on random. Normal speeds.
Set on hard or unfair for all AI players.
Primarily use capital ships with anti-fighter support at first while the planets get built up. More balanced fleets show up in the later part of the game when they can be afforded.
The main offender that i've seen do what I describe are the Advent plasma stars (I forget the name). The TEC torpedo ones don't seem to be as bad or aren't hitting 3-4 targets at once.
The AI in my game quite frequently shows up with 6-12 (on average; frequently more) of the stars and wrecks the whole system almost before I can move to the system. Thats fairly normal. Irritating, but normal. What isn't normal is now it's doing much the same thing to groups of capital ships. This is compounded if there is a fleet along with the bombardment ships as they have quite a enhanced effect on each other.
Now; your right that they are a bit fragile. But their range is such that they are hitting targets from hallway across the planet system and the capital ships are not exactly the fastest units around to close range on them. If there is a fleet rather than just a raiding group (AI seems fond of blocks of these to raid planets solo), this is compounded since to get to the bombardment ships means taking a beating the whole way to them while still under bombardment.
Worth mentioning; oddly enough I seem to mostly end up fighting Advent the majority of the time. Bit odd for random placement, but the TEC usually is occupied elsewhere.