Sewn in gives you a bathtub floor that you generally won't get with just a groundsheet.
Sewn in is harder to fold and pack, at least it was for me.
Sewn in does add bulk when packing the shelter. I hated packing that thing up with the sewn in floor.
The sewn in floor, if cuben, can drift on a slope as well, increasing the stress on the netting.
After having had both a Hexamid with just netting, and a Hexamid with sewn in floor, I ended up preferring just netting. Of course, I had one of the very first sewn in floors, so changes could have been made since then. And I had the solo with just netting, and the duo with the sewn in floor, so it's not a completely fair comparison.