There isn't a Challenge Rating, no.
Every combat is potentially lethal, since any opponent can land a Crit with a good roll, and 65 / 66 on the table is lights out for your PCs. Hell, Crits from 36+ (more than half the results) are "Lethal". This is a deadly game, and if you're playing with D&D players, you should probably let them know this up front. Running from or avoiding a fight is a good (sometimes the best) strategy.
The way I think about balance is simply in terms of numbers. If the PCs outnumber their opponents, they're likely (but not guaranteed!) to win. If they're equal numbers, it is much more down to the dice.
Gear is also very important. Thermal weapons are so OP that I advise you don't introduce them at all, or at the very least until you're well into the campaign and have a better handle on things. Explosives are likewise very deadly, and finally some of the rare armour will turn a character into a tank.
This game attempts to simulate reality more than D&D does, so telling the players to remember that may help.
My ultimate advice: focus your prep on the story (or setting if you're running a sandbox), not combat, and don't try to balance combat should it occur.