I've been working with Kafka since 0.8, I mildly beg to differ on "easy to learn and use", just based on the fact that to use it well, you have to design your applications for its semantics, and that tuning it requires a lot of indepth understanding of its mechanics.
And I've seen a looot of bad designs and misconfigurations.
All that said, I'm a massive fan of Kafka, I'm the first to admit it's a complex tool, but it needs to be for the problem space it targets.