What does it mean that mail mode doesn't support MIME?

[BP25]

I read: "Prior to Emacs 23.2, the default mode was Mail mode, which is
similar to Message mode in many respects but lacks features such as MIME
support." and also "The most typical thing users want to use the
multipart things in MIME for is to add “attachments” to mail they send
out." From this I deduced that mail mode doesn't support attachments.

However, mail mode supports attachments (use the mail-add-attachment
command to do that) and C-h f mail-add-attachment says it adds it as
MIME attachment! I deduce that attachments are in the "universe" of
MIME, and by "lacks feature such as MIME support" really the manual
meant "lacks features such as complete and total MIME support", right?

This begs the question: What MIME things then does mail mode then not
support? I couldn't find info on the manual but if you direct me to some
reference I'd be very grateful.

Thank you so much!