It's simple to use, very flexible at creating handlers for different protocols such as HTTP requests/responses or even custom ones (if you implement them).
The documentation can be sometimes tricky but it gets better over time with more examples available online. I would say that there are no downsides so far regarding my experience using this library - everything works perfectly fine as expected; we're able to build robust servers without any problems in terms of performance issues when handling large amounts of traffic. We've been building our backend API server mostly from scratch since most frameworks don't support all necessary features required by us yet so Netty was perfect fit.