In the last picture, I think the possible (!) problem is pretty visible - do you see how your muscle kinda distorts the smiley? So at least in that position, the flex would no longer be able to lay flat, and it won’t be able to follow the movement as well, so the skin would have to to stretch around the edges a bit. But I don’t know how often you keep your arm like that^^ Another thing to consider with that placement (realizing that while typing here…) - I often rest my arm on that spot, and I think until it’s fully healed, this might be pretty uncomfortable.
I’m not sure if the flexMN is more or less flexible than the flexNExT (I think it’s less flexible?), but if I would think about implanting something similar again, I would cut it out of really stiff cardboard and put it on the desired place - that way, you can clearly see how few flat surfaces a body has 
I did it with a 5-cent-piece for my considerations of a flexEM, and I found no safe spot on my hand at all, and only a more-or-less safe one near the elbow, near the spot you’re thinking about as well.
That said - I am probably a bit too careful because of my body’s reactions, and you might have zero problems with installing it on your hand or arm. So I hope some others will share their opinions as well, for I know I am a bit biased
Lucky you! My beloved one usually jokes a bit on me if I am handicapped because of mods again… well, of course he helps me, but not without making a little bit of fun of me