Tech-mining works fine and the loot is pretty much what I'd expect for the cost. I'll even colonize planets just to tech-mine 'em out. It's something to do super-lategame when money is virtually infinite. I look at it like hiring an agent to do extra exploration for me: of course it's not as effective as if I went exploring myself. But it's extra value on top of whatever I'm doing, at a game stage where money no longer has value, and accrues in the background with minimal supervision. If the planet gets raided, no big deal, the tech-mine is finite anyway so delays aren't lost equity. And when it's emptied, if I'm playing Nex, I can give the planet to my faction.