Apple Mail users: what breaks your email signature?

I'm launching Autograph here on October 20: a Mac app that installs email signatures into Apple Mail in one click.

The problems I set out to fix are logos arriving as attachments or question marks, signatures vanishing after a macOS update or iCloud sync, and signatures that never make it to the iPhone.

If you use Apple Mail every day: which of these has bitten you, or is it something else entirely? The answers decide what I fix next.