I think that it has gotten nasty because it has created a rift in the community. There are many who think that we need a new init system for Linux, that most of the init systems are antiquated, etc.
Others think that, for a number of reasons, systemd is the wrong answer, and that it is trying to be the one ring to rule them all, as it is trying to take over or tie into not only init (systemd), but udev, binary logging (journald), disk management (e.g. fsck), networking (networkd), it appears to be trying to take over Linux.
There are two other things that, I believe, makes this such a hot-button issue. First, Gnome made it a hard dependency, then distributions have made unilateral decisions on behalf of their users, primarily without consulting their users. And since most of the major distros have gone with systemd, with the exception of Gentoo and Slackware, what does that do for all of the downstream distros? Debian has, based on the last diagram I saw, has the most (by a large margin) child, grandchild, etc. distrubutions, so not doing systemd could be a major disruption of the distros.
The final item is the personalities of the systemd developers. Lennart Pottering and Kay Sievers seem, at least to many outsiders, to be trying to ram this through at all costs. Even Linus was less than happy with Sievers.
There is a page that describes many of the issues at http://boycottsystemd.org/. Don’t let the possibly inflammatory site name dissuade you, it is a fairly well thought out and well written page on the technical downsides of systemd.
Personally, I am less than happy with systemd myself, and have started dabbling with the BSDs.
Every election cycle makes me think of The Hitch Hikers Guide to the Galaxy when the giant robot lands and says “take me to your Lizard.”