Hey,
IT PRO here....
There is nothing wrong with an external graphics unit, it does have a slightly less then average performance on mac computers then it does on traditional PC's.
You will have to buy an extra component which can cost anywhere from 150-200 dollars to hook your graphics card onto and attach it to your mac.
PROS:
Your Mac can game now.
CONS:
You can face some driver struggles depending on the brand of card you get.
Buying your first graphics card will be more expensive then it would be on a traditional PC because you will have to buy the additional device to attach your new graphics card to. So when buying go ahead and add an additional 200 dollars to your initial price of your card you are going to buy.
Native games on Mac computer's are slim and scarce and have a clear lack of a wide verity of games. Why? Because majority of the games are programed to have both a left and right click feature, something that is clearly not functional in the Mac OSX more so if you have a mac branded mouse.
Overall Analysis:
It would be far more beneficial to you to go out and buy a top of the line PC like an Alienware desktop then to spend 600 dollars on the top of the line graphics card + 200 for the external graphics card enclosure box. Why? Because not only do you get the newest grade processor, newest model of ram you also get a more broad variety of games to play. Keep in mind... Not even Blizzard's overwatch has a stand alone version for the mac because Mac computers are running far behind the gaming scene.