In some ways a step up from a tarp. Tarps lack floors, a bathtub edge to keep puddles out, and walls to keep wind out. Yes, all these can be provided other ways, a watertight ground cloth can be formed with 'mattress corners' on stakes to be a bathtub floor. A tarp can be rigged low and on down to the ground to keep wind out while providing ventilation.

Polyester has some good points. Polyester is much more resistant to UV than nylon. Polyester also doesn't stretch so much, so the geometry doesn't change, allowing sag or flapping, and it doesn't lose strength about (15%) when wet like nylon does.

Most modern waterproofed fabrics use a very thin urethane coating. This coating is subject to mechanical damage, aging/decomposition over the long term, and delamination on stretchy fabrics like nylon. Polyester doesn't help with the first two but can make a difference on the third count. The good news is that urethane coatings can be replaced and repaired with paint-on sealer. Sil-nylon is saturated with silicone and so highly water repellent. Until there is sil-polyester to get the same water repellent qualities it has to be treated. DWR or silicone will work but neither is permanent. But this might not be an issue for survival and disaster use under a month or two. Longer term you may need to re-treat.

Of course nylon's tendency to slowly degrade with exposure to UV means that any extended exposure might get iffy. Early nylon tents had no chemical UV blocker and the material got weak and brittle in a shockingly short time. Sometimes a month of tropical sun was all you could expect. A lot depends on the amount of UV blocker is included in the fabric and how much excess strength is built into the tent design.