Not speaking for/agreeing with them, but vis-à-vis mail, I figure they mean the encryption/security is quite outdated on Haiku’s in-built mail app, so many modern services like GMail refuse to work with it or require warning-laden security exemptions before they will connect.
Edit: It seems GMail’s protocols are actually nonstandard, and this is why Haiku Mail has issue with it. Obviously it would still be nice if it worked, since it’s the biggest provider in the world, but still, more Google’s fault than Haiku’s.