Analytics is what Kudu was designed for (it's not just marketing), so some tradeoffs were made. You'll get the biggest bang for the buck if your use cases are heavy on inserts and big scans with selective filters. Other use cases will perform ok or meh VS other storage engines. Also note that Kudu is still pre-1.0.
Did you initially pick ES over systems like Impala because of the lack real time inserts/updates when used with HDFS?
BTW, here's a presentation that might help you understand Kudu: http://www.slideshare.net/jdcryans/kudu-resolving-transactio...
Secondly, ES is easy to deploy and manage. Being on the JVM, it admittedly has a considerable RAM footprint, but at least it's just one daemon per node. With anything related to Hadoop, it seems you have this cascade of JVM processes that inevitably need management. And lots and lots of RAM.
Thirdly, as you point out it's easy to do real-time writes.
I do like the fact that Kudu is C++.
You're probably happy with what you have in prod but if you get some time to try out Kudu feel free to drop by our Slack channel for a chat! http://getkudu.io/community.html