If the Deterministic Password Generator does not generate a valid password for a given site, it's certainly a fatal flaw for that site, and a usability nightmare - now I have to remember which sites aren't supported and keep a vault anyway.
And I guess point 2 isn't a fatal flaw until you need to change a password for whatever reason, but after that it becomes quite a problem.
I know there's a group within the security community willing to tolerate poor user experiences, or wear them as a badge of honor, "the price of security" or so to speak. But often I find these are the same people scratching their heads wondering why OpenPGP encrypted email hasn't seen more widespread adoption, or why the Web of Trust failed (or perhaps they think it succeeded), while at the same time bemoaning successful encrypted chat systems like WhatsApp and Signal.
I would argue that user experience is just as important a consideration as the security properties of a tool.
Even with a GUI like Enigmail, I needed hour long explainathons to make it work for these friends. Smart, borderline brilliant folk.
The same people now use WhatsApp with encryption and don't even know it. That's what a good crypto UX looks like.
I'd love for a google-free, proprietary-free, facebook-free alternative to exist but if the alternative is painful hours of setting up encrypted mail only to have it cease working after two weeks because the user forgot their passphrase, I'm gonna say I don't care too much.