Hi Thang,
Unleash is a great feature flags product and, like you said, is more mature when it comes to permissions. Our philosophy with GrowthBook is to feel more like an internal platform you would build yourself as opposed to a 3rd party SaaS tool. So the setup and integration is more involved, but also much more flexible.
Also, GrowthBook originally started as an A/B testing platform, so our experimentation and analytics are way beyond what Unleash offers. If you're going to be running experiments, GrowthBook saves you a ton of manual data analysis work. Unfortunately, we don't support Countly right now as a data source. They are based on MongoDB and we focus mostly on SQL data warehouses.