What did you not find easy about KryoNet? Also, have you ever tried to use Netty?
To answer your questions, I have used Netty in the past for personal projects. It was a toss up for me between Netty and KryoNet at the time.
Netty has exceptional resources and documentation, which is something KryoNet lacks.
Additionally, back then Netty was underneath Jboss so I could rely on active development with plenty of bug fixes / or support.
Netty was more actively developed.
Netty had a usage that was very logical.
I chose to use Netty at the time, it worked out great. Besides the time I was torn between the two I have never looked at KryoNet so my reply was purely my personal opinion.
I didn't consider Mina at the time because of I had heard that the API was going through serious changes from Mina 2.x to Mina 3.