The performance aspect also makes a huge difference in development. The application boots fast and the live reloading experience is rewarding. Having a 100 test cases that hit your endpoint+database running in less than a second is pure joy.
I can't stress how important that is. We have about 1.5k tests in our Phoenix app and they almost all hit the database - they run in 30 seconds. I compare this to a Rails app where 1k tests run 8 minutes. Having a fast test suite makes development so much easier and makes you rely more on tests.
Urg, I'm so sick of waiting for rails to boot up. It takes like 30-45 seconds at work...