People who oppose SystemD dislike the fact that it takes on two much responsibilities and unneeded complexity as an init system. People who like it accept the added responsibilities and complexity because it has widespread adoption and works .
I think the main problem is tight udev and kdbus integration with systemd, and applications that use either of those components implicitly depending on systemd.
That, and systemd having large scope creep, absurd ways of dealing with bugs ("this old system is broken because it does not use our interface"), and generally behaving in a dominating and lock-in-promoting manner.
They disapproved the development model like vacuuming too many linux parts, and the design, many people said you could replace systemd with other simpler and more unixy bits like djb rc (IIRC).
(By the way, the mis-spelling "SystemD" has become a shibboleth over those years. Spell the name properly, lest you find yourself lumped in with the pot-stirrers and provocateurs by the rest of the world.)
There was a whole hoo-hah in Debian over what to replace System 5 rc with: upstart, OpenRC, systemd, or something else. It got raised to the Technical Committee, sparked several resignations, went to a vote of the entire Debian Developer membership, and lasted for a long time.