I seem to remember this being an issue when I took yoga. Just tried it now, and I can’t do it flat-footed at all, even leaning against the wall. It feels like the main problem is lack of tendon flexibility, but there’s also a lot of pressure where my calves and thighs press together (I’m short and kind of chunky, so I’m sure body type has something to do with it). If I try to lean back a little to relieve the pressure, my center of gravity goes too far behind my feet and I topple. I think this could improve a lot with regular stretching though.
The other yoga issue was the sitting on my heels. I can’t do it for more than a few seconds - the pain in my knees is excruciating. I don’t have any knee problems otherwise, though.
ETA: My brief experience with squatter toilets (in Italy) was surprisingly non-traumatic. I didn’t feel it was difficult at all. But then again that was a decade and 20 pounds ago, so that might have made a difference.