I had an XD9SC with a grip safety. I kept managing to depress it while holstering....
I have an M&P9C with a thumb safety. IMHO, the thing is too big to hope to avoid accidental disengagement, although it would be unusual.
Note that most striker-fired guns
could be discharged by trying to hammer a nail with it if there's no "drop safety" such as the ones that S&W uses on the M&P pistols. Many years ago, I had a little Browning semi-auto in .25ACP. No drop safety.... I really should have kept it, but that one could fire if dropped.
The thumb safety on the Shield is a bit too stiff for my liking, but I'm a 1911 guy, so it's not the end of the world.
(I could remove it easily enough, but I'm a believer in not removing factory-installed safety devices.)
(When I bought the Shield, I couldn't find one without the thumb safety.)
Guess I should buy another one
....
Regards,