There are 2 reasons only you should use a Mac for Game development is if you are purely targeting OSx / IOS as your only platform, otherwise, you will need a Mac to build for that, but even a MacMini qualifies for that.
The second reason is if you allready have a Mac and can’t afford to get a pc just for development.
Otherwise, if you have a budget and are getting into Game Development, buy a PC with a Nvidea Graphics card.