I have two of these that I use for my orchids outside and I really like them. In fact, I just bought a third one to have in reserve.
For most of the year, I just use the shelf units, and put plastic jugs of water on the bottom shelf to keep them from blowing over. The jugs also retain heat to keep the plants warm at night.
They're lightweight, and not terribly sturdy, but good for seedlings, small plants and orchids which are pretty light as they don't grow in soil. The plastic cover for the first one tore along one zipper, so I had to sew a piece of stretchy fabric in there, you can see it in the picture. The second one seems to be made of more flexible material.
When I do use the plastic covers, usually just for a few cold nights here and there through the winter, I absolutely make sure to unzip them, or take them right off before the sun hits them. I'd never leave the house with them zipped up.
You have to do this with any greenhouse, if it doesn't have automatic thermostatically controlled venting.