cssnext has some features that LESS doesn't like auto prefixing CSS rules according to your browser scope. So it rely on caniuse database which is why it's not a good idea to use it on the client side. That being said, there is a playground so you can try it http://cssnext.io/playground/
SourceMap is supported, see options http://cssnext.io/usage/#sourcemap
Does cssnext support source maps? This is an important feature for any non-trivial project, as it lets a developer inspect how the rules in the source language are translated into the rules read by the browser. Support for source maps in Chrome Developer Tools makes a workflow much more efficient: https://developer.chrome.com/devtools/docs/css-preprocessors