I just played a solo game with the Alien Destruction cards in. I added 3 copies to see if it felt right. It was hard, and they are so annoying, but it was doable (and probably easier than double alien meeples), so mission accomplished.
I did, however come up with more balance issues. For one thing, bad draw luck made the final two cards in the part deck the two I needed to defeat five of the first six aliens. That made for a really rough start.
I think that I need to genericize damage in some way, perhaps make any two attack symbols count as one other attack symbol of your choice.
The mother ship was an amazing fight, and also way too hard. I had nearly every tech researched, but I still couldn't defeat her without blowing up parts.
The types that the mothership required were acid fire and lightning 2x each, and there is no weapon that provides that, so I had to use 4 different weapons to get it. I ended up building a monster of a robot that had practically no healing. I blew up three parts, but did defeat her in the end.
New open questions:
Why are there no better researchable repair units?
Should I add wild card weapons? One time use weapons?
Should I clean up the way the aliens move?
Should I add a zero space reactor?
I will stew on this and then post later...