I was frustrated with all the validation libraries out there. They all are hard to write custom error messages for, most of them are schema-less, they are not interoperable between frameworks, they did not do asynchronous validation, and many many more grievances. So I made my own validation library data-guard. If it does not work well for your use-case submit an issue. Let's work together to design a simple, extendable validation library.
If you want to learn more about data-guard, bug me about it on the issue tracker and I can write some documentation.