It's a mouse, not a handrest. I think that's the problem with everybody claiming the MM is not ergonomic, they're trying to rest their hand on it like they do with one of those crappy asymetrical mice. I had to look up the MX Master 3, and if I ever had to touch one of those it would be going across the room and as far through the wall as I can throw it. It's not as evil as those idiotic shark fin mice (you've seen them, they're advertised to help carpal tunnel, they keep your hand turned sideways. Those and trackballs are part of why I always have a small wired mouse in my bag, gotta have something to plug in when the user has an unusable mouse.)
If you're using a mouse properly, you'll never feel the shape of it on your hand, because you're not resting your whole hand on it.
Logitech has always been a mixed bag, over the years they've built some good mice and some bad ones. That MX Master 3 is one of the worst of the worst from them. Apple has only ever built one bad mouse - the hockey puck. No surprise that Steve Jobs was partially responsible for that idiocy getting out the door. But even the hockey puck could be fixed pretty easily, just remove the colored plastic bits and it's not round any more.
TL;DR: If you think the Magic Mouse is not ergonomic, and not in fact one of the most ergonomic mice available, you are, in fact, holding it wrong.