Honestly you can find most of those answers with the search function and I'm not going to reinvent the wheel here for you.
Let's drill down a bit. What are you trying to accomplish? Do you just want "different" or is there some problem you're trying to solve? If your goal is to be as good as you can be, pick one platform and stick with it. If you want to collect or have a new shiny play-toy buy whatever catches your eye. Can you deactivate the safety 100% of the time when under extreme stress, possibly being grappled, and possibly injured? If you're willing to bet your life that the answer is 'yes' get something with a thumb safety. If that's not a wager you're willing to accept, get away from thumb safeties and go with a TDA or striker fired gun.
As far as why I wouldn't want one, any semi-auto I can fit in a pocket isn't going to shoot well enough for me to be satisfied with. The Shield is as small as I can go and not feel like I might as well hold the gun with chopsticks.