Having the box in hand, I can tell you really the main thing against solo-play, is how do you determine "victory"? Some threshold of DF tokens to hit? Even if injured, theres never anything that can make you actually die or unable to leave the facility. The big issue might be getting too far inside, let the proximity meter get up there, and then be injured with not enough time to escape.
I did play a 2 player game "against" myself, but that doesn't really count. I will say the two player game is quite interesting (given how it changes up the turn order with operatives, each player controlling two and what not).
For solo play, I guess the big thing would be to set a threshold of target DF points, based on the amount of operatives you choose to field.
If you can choose a good target value, then you certainly have a game you can play solo. The two player rules already account for a notion of a single player controlling 2 operatives (separate item hands, DF pools, etc.). Just play the game as normal, and add up all DF tokens at the end (minus the ones for any operatives who could not escape). Some item cards may need to be removed, as they serve no purpose in a single player game (anything that raises the proximity level, or causes operatives to be delayed).