I have no idea how/why it is hated. For me it is one of the best open source server packages. For instance if you have a specific problem you will find some configuration directives on the Internet and they will actually work.

OpenSMTPd seems easier superficially but it's not particularly well-documented. I don't remember the details but I remember struggling with setting it up because certain config flags were not documented and I had to guess their exact meaning from the few blog posts out there that discuss OpenSMTPd. (It also didn't help that there were slight changes in the config format at some point in the past.)

if you started with sendmail (and sendmail.cf) then postfix was alright

That's it, also Qmail was a nightmare (still is?) Perspective most of times is everything.

Oh I miss qmail, felt much easier to deal with than the alternatives.

I’ve been using it for a decade without issue. Only problems I’ve ever had self-hosting were related to being randomly junk-black-holed by the big three independently of each other for 1~30 days for seemingly no reason, but no software is going to fix that.

User experience with it, and opinions on it, seem to vary. I, for one, rather like both postfix and dovecot: both are well-documented, maintained, lightweight, reliable, yet configurable and feature-rich software, with few dependencies and good track records.

haters gonna hate

Probably popular because it’s the default mta in Debian-based installs.

Hey it could be worse, at least it’s not sendmail.