I've been carrying an M&P40 Shield for the past 2-½ years and am perfectly confident with it.
The M&P Series of pistols are extremely safe to carry because they have multiple built-in safeties from the trigger safety which prevents it from being pulled by mistake, to the firing pin block with prevents the striker from impacting the firing pin unless the trigger has been pulled, as well as an optional frame mounted safety switch which prevents the trigger from being pulled fully to the rear while it is engaged.
The M&P Series is a precocked Double Action Striker-Fire design, ergo when charged, the striker is partially cocked, requiring a relatively short, light trigger pull to fully cock and release the striker. So it's sort of a hybrid between DA and SA since the striker cannot be fully cocked by pulling the trigger alone, yet isn't fully cocked when charged either.