Trying to explain it again:
My wife suggests emphasizes: You don’t freeze in or after the cryo! In those 2-3 minutes your are numbed. And after that you are warm, because in it your blood vessels have become narrow (vasoconstriction) and afterwards they become wide again (vasodilation). So you quite to the contrary “warm up” by doing it, and do not flare. (To feel safe you could take something warm for afterwards with you, like tea or pads).
Cryotherapy for the Ache is like - actually better than - ice cream in the burning sun 
or cooling your wrists & arms when it’s hot: it numbs and soothes the pain. It really is not that uncomfortable - it’s by far not as bad as eating too much ice cream in one go, more like a winter walk.
My PT (cryo&acupressure) compares it with that. I agree only is as much as the cold creeps on you slowly in both, so the shock is not as big as with cold showering. She also means: for non-fibromites. But it also creeps out of you very slowly after a long winter walk, I can remember, when I used to do that before fibro, of sometimes needing hours to feel OK again. (Now I don’t risk it too much, even tho my cold tolerance is better. I never go above 90mins., better 40-60’. And only 20-30’ if I’m not feeling too good.) After the cryo you feel warm pretty immediately. After a cold shower you do too, but I sometimes need a hot water bottle for me feet. Yesterday I cold showered at 12:20 and then did cryo at 1:10 - that was completely OK for my body, but my feet had to be packed in warmly after and remained slightly cold…
Cold showering is tougher than the nitrogen-cold air in a chamber or barrel. And you get used to it. In the rheum./fibro clinic I could do it 2x/d, but unfortunately it was much “warmer”, so not quite as effective. But just think: After 3 minutes you come out, with no side effects, you warm up immediately, but can often suddenly jump like a young deer, have energy, almost no pain, feel healthy again, at least for a few hours, sometimes days: Why wouldn’t you do something like that??? 
Cryo does freeze … or better numbs my nerves, exactly what I need. And not just the pain, but clears my brain day at night, so I can sleep again… And uncomfortable? About as uncomfortable as the prick of injection. And FAR less uncomfy than the side effects of most of the fibro-meds I’ve tried. Only our bodies don’t realize and don’t associate them with the darn pill-taking. Needs quite a bit of head work…
And as I say above: Getting too cold used to be one of my absolute biggest triggers! Oct2019 I flared big time after having to stand outside at 17°C/63°F for an hour. My first time was too cold -130°C, which hurt a bit, so I went up to -110°C, altho it was OK after, but then went down again bit by bit to -150°C as I got used to it. So it took about 3x to realize that & how much it was helping.