@Martin no problem for me so far with vuefire but my app is far from complete. It’s true that, just like with any other nosql database, relations are painful.
I really enjoy the Firebase authentication system, although it was bit tricky to implement a complete solution with usernames. Unfortunately they don’t care about i18n (english only password reset page etc.). The free hosting is something to mention too.
Just like you I had a look at gun.js in 2016, and keeping an eye on it