A stainless Watch with milanese loop will be a fairly hefty combo. You'll definitely feel it on your arm when running, it might be distracting having a weight on one of your wrists.
Also, the constant motion and the heft of the watch itself might make the magnetic clasp loosen and make the pulse sensor lose contact with your wrist. *shrug* I haven't used the milanese loop, I only test-wore it (upon which it pinched my arm hairs, ugh), and it fit very securely in the store I thought, but I've no idea how it holds up in the long run and during strenous exercise, and especially if the band is drenched in sweat.
Why not, and if not this combo, what do you recommend?
I suspect it's because stainless metal gets cold when the air is frigid - having cold metal constantly in contact with your skin might be uncomfortable. If you wear a long-sleeved sweatshirt or similar while running it wouldn't be a prob tho.
Also, again - arm hair pinching. Maybe not a concern for you, I dunno, but I can't use metal armbands for any reason. I hate the sensation of suddenly and randomly feeling hairs on my arm getting pinched, it's a sadistic form of torture IMO...