What tells you he'll be any good? To me, his sample size is way too small. There's a stat out there that QBs with very minimal college starts mostly end up being shit in the NFL. It's extremely possible that Simpson will blow and I personally wouldn't be that surprised. NFL is a whole different beast than college.
What tells me he'll be any good? My answer would be his brain, his arm, and his legs. With proper nurturing of course.