I’ve complained extensively about Firebase in other posts, but not about their Auth module which I think is by far the best offering in the portfolio of services. I’ve integrated it for a large enterprise customer through SAML and have had an extremely positive experience where everything just worked with about a morning of work, and I’ve not had to maintain it ever since. The client SDK is great in how it syncs auth states with the server.
My main complaint is that there is no ‘onLogin’ serverless functions hook, and how keeping additional user context through custom claims is not that straightforward.
And of course I’m pretty apprehensive about Google having the data, so best of luck to you all building OSS solutions.