1. obviously its open-source !!
2. Built on Node.js they are providing support on par with enterprise softwares plus they have a SDK for almost every platform that present day tech company uses.
3.I am following Countly right from their 3rd release on github there are many improvements actually their team are actively taking feedback and suggestion from the community to build the software.
4. They have free tier for their hosted version so developers and students who wants to contribute/use can easily get up and running.
Review collected by and hosted on G2.com.