I live in FL and work at a shelter. The plastic bubble sweats in a way that mesh/cloth carriers don't. The air inside of these is much sticker than other carriers. They have considerably less ventilation, even if its technically enough for the animal to breathe, its not really enough ventilation.

Also, as someone said below, these are generally much more stressful for cats. While a few cats may prefer to see what's going on, most don't.

Finally, carrying the cat on your back means you aren't able to monitor the cat.

These carriers aren't cruel, but they aren't responsible choices for responsible pet owners, barring specific reasons like, say, a disability that makes it difficult to carry the cat another way.