Floor sleeper here. Pic related. That's Momiji on my "bed" I've slept on for the last 20 years. Yes, that is a single 3'x6' tatami mat. No, I do not sleep on a futon. I sleep directly on the tatami.
Can confirm it's great for your back, breathes better than any mattress, and is extremely durable a la I bought one 20 years ago and it's holding up fine.
For context, I sleep on my side but presumably there'd be no issue sleeping on your back. I meditate that way and don't run into any circulatory issues.
All that said, I do occasionally get circulatory issues if my arm is pinned under my head wrong. Just this morning I woke up and my left hand was numb. I'd say that happens once every other week.
The other potential downside is once you get used to it, normal mattresses feel wrong and I often find myself sleeping on the floor in hotel rooms and most carpets are not as comfortable as tatami. I recommend putting a sheet down first.