Does it also mean that the mimemail solution is not adequate for large lists on shared servers?
Yes, that's correct. Mailing list managers like EZMLM and Mailman have many optimizations built in in order to handle large amounts of mail. Sending via PHP's mail interface (from Drupal) will likely run into resource problems.
The only problem is defining exactly what a "large list" is. But at some size and larger, mimemail will not be adequate.
..chrisxj