It is cool but useless for most of the users. People connecting a real vario already have sounds. People using lk vario derived by gps altitude differences are seen 2 seconds late and inaccurate measures. Adding sounds requires to create the sounds and buttons and configurations. Depending on the results you want to achieve everything can be done in 5 minutes or in 5 weeks. But my main objection is about latency and generally the inaccurate measures compared to even a basic low cost vario. Maybe for an average 30s vario it could work. Again lots of testing and configuring. Useless for most people..
More phones have barometers now, and brighter screens, it could be a really nice 'all-in-one' solution. I will test a Xiaomi Mi8 in the spring, I'm hoping it will be as good as the reviews of the screen suggest. If it could also produce vario sound it would be a perfect backup instrument in case of failure of the main vario (I had this happen).