As previously mentioned, one of the biggest gripes with PUBG Mobile is the controls.
As a result of not having the fine aiming capabilities of a mouse or the ability to swing a stick around to see behind you, if you get caught from behind, you are screwed.
There's just no way to effectively turn around and take out the person gunning at you in time.
Because of this, you should always be trying to get behind anybody you see if the opportunity presents itself. It's going to catch them off guard and almost guarantee they die before even having the ability to turn around and return fire.
PUBG Mobile just really doesn't allow for fast movements, as players often need a couple finger swipes to turn around, instead of a flick like on console or PC. This is just one more thing that can give you a major advantage in a gunfight.