Plastic (and, to a lesser extent, ebonite/hard rubber) does not "move" like wood. Even a high-oil content wood like grenadilla is subject to this curse, and you always face it as an issue.
"One piece" clarinets are not any more susceptible to cracking than others. However, with a severe crack (and such things do exist - you get some doozies with oboes that can run over half the length of the touchy upper joint), the one piece clarinet is toast. The same horn in two or more pieces only loses a single joint.
And, I know that small cracks can be fixed, often to the extent that it is as if the crack isn't even there. But, I'm talking about awe-inspiring cracks - rare but catastrophic.
And, as almost all of the unibody Bb clarinets are older horns, the age factor (and future potential for such damage) has to be considered as well.
If the horn cost $50.00, it would be one thing. However, I doubt that the vendor is offering it up for a single General Grant.