* Way too many front-end or full stack projects to choose from - even if there are many backend projects, the ones highly dominating the spectrum are the front-end ones. Bad if you are a backend guy.
* Can take so much time between project and project, depending on the client, the client & Toptal interaction (paperwork and similar), or the client can turn you down in the middle of the interview process.
* There's a slight tendency to advise developers to keep their salaries relative to their region. This is not enforced but I was told a few times to decrease my fees as they are 'too high for your city/country'.