1. There's no two-side agreement - after you post a gig someone may just order it without contacting you first. That may cause arguments with clients that made a mistake in the order/ did not include extra pay for some extra service they want, ect. That may also cause clients making orders and then cancelling a few minutes later for no reason (they probably found someone cheaper.). In this case, you're rating will be harmed and you might even get your seller level demoted (this happens A LOT).
2. They take a big cut of 20%, and also other transfer commissions when you want to get your payment. Personaly, I don't find this service to be good enough to take such a big cut.
3. They have THE WORST costumer service. All they do is deny that there are any problems and move the responsibility to the seller. I've spent much time corresponding with them (too much time, since I need to repeat myself and bring them more and more proofs for the problems I found, which they try to deny exist... It's basicly a waste of time even reaching to them - they wil waste your time!).
4. This is a platform made for buyers. Most sellers will work here for much lower profit than in any other companies Iv'e tried working with.
5. Remember - this is not the way to get to the big fish, this barely yields an extra side income... So if you use it don't put to much effort into it. Selling yourself short is never the way.