Gaps between projects. You should be ready for 1-2-3 month gap between your projects, so you have to do savings if you want to work only on toptal.
Pure legal support. The contract becomes worse and worse after every iteration, a legal team gives no support if you have issues with authorities. It could be very hard to prove the legality of income with a current toptal contract. Because different countries could have different requirements to international contracts. It was much better two years ago.