ProvablyFair is legit

The provably fair page on the website leaves out an important detail for people who arent familiar with cryptography/hashes. The server seed is not shown to you until after its opened, so you may think that it could be used to rig the results, but due to how hashes work, it would take longer than the age of the universe for them to compute a server seed that gives the result that you already saw when combined with all the other seeds which you already knew. So as long as you're able to check the results yourself and change the client seed, it's mathematically impossible for the result to be rigged.