(I really need to at least get my hands on a Shield....)
The thumb safety on the older M&P's is very easy to engage or disengage. There are mods to change that, but it's probably best to train for switching it on and off if present. I sure wouldn't trust it in a pocket. Best would be a proper holster. I still can't remember whether the one I'm using for the M&P's was purchased for them, or for an XD9SC, but it works & fits just fine. (Leather - YMMV with Kydex.)
It
looks like the Shield's thumb safety could be trusted to stay wherever you put it, but I'm a teensy bit uncertain about that.
(I found that the nominal value of the grip safety on the XD9SC was a joke.... When holstering, I tended to depress the grip safety.... Depressing?
FWIW, I liked that gun. Just liked the M&P more....)
Regards,