Yes, was just going to comment this...
Worth mentioning that its back-end app[1] is also open source, not just their Android[2] and iOS[3] apps. So basically open source + custom domain = no lock in. But I'm not so sure how easy it is to set up your own back-end server (I imagine it's like setting up your own email server, which isn't trivial)
[1] https://github.com/simple-login/app