A properly designed trigger system for striker pistols... Really makes them safe without a safety. The Glock is one of the safest designs... It is basically physically impossible for the pistol to fire unless you pull the trigger. No amount of banging, or drops, or anything else with make it fire. Only the trigger being pulled to the rear.
A manual safety really doesn't add much except peice of mind to certain people. At least on something like a Glock. I wouldn't carry a 1911 without one.
If you can make sure to take the time to look and make sure your holster is clear when hostering the pistol, you remove like 90% of the potential problems.
If you liked the Glock trigger well enough, you would likely really enjoy the VP9 or PPQ triggers. The M&P trigger is ok, but to me the trigger bow flexes too much, so I replace that with a stiffer version. Then the trigger is pretty good.
If you really want a safety, the M&P offers them. The M&P 2.0 has improved the trigger pull a good deal, but it still flexes a bit. With some drop in parts, you can get the M&P trigger to be very crisp and smooth. Something more akin to a single action hammer fired pistol, just with more take up. With a manual safety, you could still be safe with a reduced travel trigger and bringing the pull weight down to 5lb or so.
Another pistol with a safety is the FNS series... The trigger is very Glock like, it's actually the same basic concept executed in a bit different manner.
When it comes to aftermarket parts on a pistol for defense use... I go by the addage of... Don't add any obvious external modifications that will catch the eye, don't add slogans or quotes that are about killing or judgement, don't remove or disable any safety devices, and lastly... do not get the trigger pull out of factory specs. (Factory specs meaning, if the manufacturer of the pistol states it has a 5-6lb trigger, don't go below 5lb. The M&P comes in a Pro model with trigger specs that are lower than a standard, so that can give you some wiggle room)
The likelihood of some internal modifications being used against me... Is pretty low. Unless the trigger is super light, most forensic guys examining the pistol are not likely to comment on it. And most prosecutors will not be able to tell much from a basic look at the pistol either.
But if you have obvious modifications to the outside of the pistol, like stippling or whatnot, they may get the idea to look harder, or have an expert look it over.
Having quotes and sayings on the pistol may be a bad thing as well, if you have things that are about killing or judgement of the wicked, they may use that as an argument to say that you want to kill people that you judge as being bad.
It's still all unlikely, and mostly the danger of such things is regional, based on the political climate of where you are. So I look at it as being a situation where you use common sense, and be reasonable with modifications. Don't draw attention to the firearm in question as being anything but ordinary, and you will most likely be fine.