TL - what that fails to discuss is slippage issues. You won't get much (if any) on the ES and you can get considerable slippage on the YM depending on the size you are trading. The size/slippage argument may be minimal if you are not trading size, but if/when that happens, you'll quickly realize the YM simply won't cut it for this reason and this reason only.
What 'pros' are recommending the YM over the ES? If these pros are trading any size whatsoever, they would not be recommending the YM. Perhaps the YM is recommended as the 'kiddie pool' but sooner or later (maybe) you'll want to swim in the deep end.