Do cheap pop-up and PE-cover greenhouses actually keep plants warm?

Across the reviewers we read, the consensus is that lightweight PE and pop-up greenhouses are season extenders, not heated structures. Multiple r/gardening and r/Greenhouses posters note they often run only a few degrees warmer than outside and primarily block wind and frost; one widely echoed warning is that they can also overheat fast on sunny days, so venting matters more than insulation.