A fun concept, presented in an actualizable manner. The mirror universe is evocatively depicted and the big martial threat is both creative and horrifying.
My two gripes are that the writeup is presented in long, wide paragraphs which make it a bit harder to pull information from than is strictly necessary, and that I didn't see anything in the mirror universe that would tell me as a player "Alright, mission complete, go exfil." I think I'd be inclined to continue deeper in if I got as far as the Radio Wraith, and there's not quite enough here to support long term expeditionary play ala A Red And Pleasant Land or such.
Worth getting, for sure. The FIST community is better off for this existing. I'd print it out and hit it with a few highlighters, or make cheat sheets, before bringing it to the table though.