I checked the Shield out when it first came out and decided the difference in size and weight was not enough for me to give up carrying my 9C.
It's my belief that the only reason these guns have a manual safety is so S&W can sell them in certain restrictive states. It's not necessary. My family owns four M&P's without manual safeties.
Simply point and shoot. No forgetting a safety under stress.
If one trains with their firearm there should be no forgetting the safety is engaged or not no matter how much stress one may be put under.