Cascades Can Kill You:
There are cascade freezers in common use in labs and blood banks all over the world. Are the people who works in these labs in danger? No. Why? Because those freezers were designed, built, and serviced by people who know what they are doing.
Do it right, and it is no more dangerous than any other refrigeration system. (Okay, maybe a little more dangerous).
Do it wrong and it is a bomb.
I see WAY too many totally clueless noobs talking about building cascade systems. This scares the hell out of me. Someone is going to die.
Frankly, I am torn between teaching you and discouraging you.