So they're 1 space for 250 attack on a planet, and my standard mine fields are more efficient than that.

However, since they're unique structures, am I right that Sabotage and cluster rockets won't destroy them?

Should I keep them and put them on my more valuable planets because of that, or are they weak enough that I might as well just scrap them for the Complex Parts?

Yes, being special buildings makes them immune to hostile destruction.

As for your choice of buildings, that's really up to you.

Yeah, it is a tough call; immunity to destruction would make them useful for important planets, but their stats are low enough that they might still not be worth it. I wouldn't scrap them 'cause I like having special things, and you can get plenty of Complex Tech Parts at some point, but I probably wouldn't recommend using them on your best planets; probably just put them on fairly useful planets but not your best, if anything.