Oh yes it has. Try sending a rich text email, for example to yourself. Now open it in another email app such as Outlook. The text is unstyled, unless you have explicitly styled it yourself. If you view the email source, you'll find a big chunk of messy HTML and styling in there, but it doesn't wrap the text it's supposed to style!
You won't notice it if you open the email in Mail.app since Mail automatically styles any unstyled text using the message defaults in Preferences.
If you however bring up the font panel and style the text manually before sending, it will be styled correctly, but only if you style it different from the default settings. I.e. if you have specified Helvetica 12pt as your message defaults, you need to style it as Helvetica 11pt. If you try styling it as Helvetica 12pt, it won't get styled at all.
This issue has been repeatedly reported to Apple for many many years, not only by me, but so far it's still in there.
For personal email I don't care as I generally sent plaintext email anyway. For business email it just looks unprofessional.