Special thanks to SAVID EDUARDO http://www.savideduardo.com/ for this Advice!
In practice, no contract can really stop someone from screwing over an artist. In actuality, the majority of alleged rip offs are made over contracts that were usually provided by the scammer and signed by artist(s).
No contract can force one party to effectively make payment, nor the other party to make a deadline, especially internationally. What contracts are useful for, is in establishing who is and/or will be the owner and rights holder of the intellectual property generated within the project, in case the project becomes successful someday.
If you are the one who’s running the project, there’s nothing wrong with offering deals that include back end profits, payments after publishing or work "for exposure". Actually, that's practically paradise for you. You get to have all of the artwork done, and you don't need to be committed to how much you'll be paying. Whether the artist ever gets paid or not, you will have already a fully made book of your property.
If you are the artist, or part of the creative team, you can be screwed on each of these points whether or not you signed a contract:
1) Back end profits - How much is that? Nobody knows. How the artist will ever know how many copies of the book were sold? Is the percentage of profits gross or net? The artist cannot make back end profits on a book that sells less than five thousand (5,000) copies. Only mainstream and well known indie publishers can reach such sales. But also, working with publishers, the book needs to sell more than ten thousand (10,000) copies in order of make any kind of back end profits.
2) Payment after publishing - If they can't pay for the artwork when it gets made and before being published. Why would they suddenly pay after the work gets published? Actually, if the book gets published, where is the need to pay back the artists?
3) Giving up your page rates "for exposure" - The artist is not only giving away their artwork, but trusting their aspirations and economical situation to an unknown source. That makes the artist totally vulnerable to being manipulated, usually because they are too inexperienced to know better.