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Thumb Safety. No, the problem with a Thumb Safety is it's a Minor Motor Skill the way your brain see's it. Under a stressful situation you may loose all minor motor skills. All my guns I train with and I train consistantly, do not have any safeties.
Put your thumb below the lever and recoil and excitement can bump it ON. If you practice the technique it's second nature and just fine, but few do. The M1911 needs a thumb safety (and all that practice and specific thumb on top of lever grip). The S&W M&P does not.