It requires building the structure under the greenhouse specifically to support the weight of the greenhouses themselves, plus all the plants, equipment, substrate, and water, plus workers.

Your average existing large-footprint building simply isn’t built to hold that weight - maybe a couple feet of snow - and would need to be heavily remodeled to accommodate it. It also adds a lot to the cost of building new structures, so it’s a sizable investment in either case.

Idk if you’d call that a downside, but that’s why this sort of thing isn’t everywhere.