First of all, using 5V GPIO pin you are bypassing input fusee that is on the RPi. This means you have no protection in case you make some shortcut etc. So If you do, a lot of current (depending on how much you power supply can supply) can flow through your Pi and can damage it. You are also bypassing TVS diode so you are basically without any protection.

Also remember that you have to put regulated 5V to this pin. Most power adapters won't output regulated voltage (unless stated otherwise on them) so you would also need to use some voltage regulator circuit. 

And last thing is that there is some smoothing capacitors that makes power more stable so it would be wise to connect some capacitors yourself.