I've found a Doom blueprint on the 3rd vast ruins planet I tech-mined, plus alpha-cores and other goodies from first 2 vast ruins planets.
You can always do tech-mining later when you are richer, once you have 2-3 colonies but don't have 6 planets colonized yet, and no longer worried about income, go out and tech-mine all the big ruins you can find. Then set up your 6 planets colony.
If you are trying to do tech-mining early game hoping to strike big when you are still worried about income then of course it may not be "worth it" depending on your luck, so don't do it early game, do it mid-game or later.