There's a few..
If you're going to use dot.tk or anything similar (especially other services from Freenom) then you should be aware that on free domains they will grab your domain if you have a LOT of users or / and requests, and they will accept any report against your domain (false or not).
So, if you do decide to take the free domain route then you may need to be aware of that. When you do get enough money out of AdSense, I would recommend getting a premium domain to avoid such troubles and help gain more.
Source: Experience and a friend.