One possible downside to qmail is its high efficiency at the cost of
bandwidth and overloading of remote SMTP servers. When I followed the
qmail mailing list, a few people complained that qmail opened a
separate parallel connection for each recipient, on each host, for
each mail message.
For example, if qmail was distributing a message to a list on which 50
members had accounts on the same machine, qmail would open 50
concurrent connections, and send a copy of the message over each one,
rather than using a single connection and multiple recipients.
Apparently this can overload a sendmail process quite easily; people
complained that use of qmail could even be interpreted as a denial of
service. There is a concurrency limit that can be set in the qmail
config files, but as I remember, it was relatively high.
Note that this may have been fixed in recent versions of qmail.