The main benefit you mention is the biggest disadvantage of this solution as you may need to scale it as traffic grows. You can avoid it by using S3/GH Pages.
I actually found that GitHub Pages didn't scale well at all -- during peak traffic, response times slowed to over 10 seconds. This was my main motivation for the switch. Nginx (even on a micro EC2) should be able to handle very high load without the need to scale. S3 is definitely a viable option as well (and I'm not suggesting you shouldn't use it) but I find that an EC2 was a better fit for my needs.