The problems that I actually experience during daily use is:
- No proper display if Unicode characters (Emacs and Gnus do this right,
VM doesn't).
- Mysterious handling of message attachments (some are displayed inline,
some are displatched to external 'viewers', but most are offered to be
saved to disk). This may be a problem external to Emacs.
Moreover, VM provides some nifty features that I haven't yet found out
how to do with Gnus:
- Automatically infer the name of the file to save a message into,
depending on the sender name (and using the mail-folder property in
the BBDB, if any).
- In the summary, display the name of the recipient if the sender is me
('me' being a list of mail addresses). This is particularly handy
since I save BCC's of outgoing messages in my primary mailbox.
- Automatically (or on demand) change my from, reply-to, selected
headers and signature depending on which of my email addresses the
message was sent to.