They do damage equivalent to fighters at ranges further than anything but dreads, bombards, and turrets, and can run from fighters, bombers, and even frigates fast enough to kill equivalent-sized groups without taking too much damage.
In close range, they'll kill the enemy no problem, but will die as well. However, they're the best as artillery and bait-n-run.
While I'm at it, the higher-mark z-mirrors gain ridiculous hp. I can, upon starting the game, max out z-mirrors and have over 100 units with 900k hp running around, doing mk 3 fighter damage.
And while I'm at it, mk2 and mk3 force field bearers gain like no hp. You can see their simulation stats hugely drop against every other unit in the game. 148k hp at mk 3? Hell, bombers have more than that; fighters have almost that. How can it defend other units when it's made of paper itself (not to mention the massive bonuses bombers, electric shuttles, raider starships, and some cannons get). mk 1 forcefields are just fine, but they don't scale well at all. There's no reason for a mk4 defender unit to have less hp than a fighter.
And on closer look, only mk 4 z-mirrors have blade immunity. Sorry for the mistake. >_>
I'm playing with only Z-mirrors right now, which is what's making this post continually update.
Z-mirrors actually out-range MLRS turrets (every mark). That's one less missile source that affects Z-mirrors.
Found fortress. Wiped out several hundred z-mirrors.
I can outrange, outspeed, and annihilate every starship in the flagship line (except spire). I just took down two flagships and a zenith starship without losing a single ship simply by flying backwards, since z mirrors ever so slightly outspeed them. Leeches are also trivially outrangable, although they can catch up after a long while (imagine crossing 13k at 2 speed), and don't have much health. Raid starships... commit suicide. Literally. They'll take down 2, maybe 3 z mirrors before imploding.