I have NEVER had a polo come undone or loosen, even when they have gotten wet, or we have ridden through brush, and other 'catchy' things... I even have a 'large' pair, and they work fantastic as trailer 'boots'; I can never seem to get normal trailer boots to stay on well enough, so have given up on those! So I have a pair of polos that are meant for larger horses, and used them to haul my girl here to CO, and they stayed put, and when I unwrapped her, she really wasn't warm under it, so both bonuses I guess. I think for normal everyday exercise, Polos are just fine...I've used them for years, and my horses have never developed splints, or any other issues that can occur from being worked without protection on the legs.
That said, I have regular smb's for her front legs, for 'harder' work, than just our normal routines. I plan on investing in an all around set or atleast new rear ones, so when I start going on long trail rides this fall, I will be able to use those instead of the polos.