Qmail has also been very resistant to security attacks. It has not been unassailable, but it has fewer security related updates than sendmail. One example of qmail's approach to security is that only two qmail applications run as root. Qmail's disadvantages may not be significant to some. Qmail has good support but does not have sendmail's broad usage or pool of experienced administrators.
Installation requires a number of manual steps that some sendmail installations automate.
The job is not extensive and the instructions are only a couple of pages, but new administrators may shy away from this. Qmail's documentation is also less sophisticated than sendmail's. There is no one authoritative source for all its various combinations, though one is in the works. Still, experience and documentation available for add-ons is often tilted towards sendmail. While both are excellent choices for an MTA, from my perspective, each has a place.