It hinges upon the fact that people are more likely to be vocal when they don't like something than when they do. Consider online reviews: are you more likely to review a product when it's worked fine as you expected or when it died a month after you got it?

systemd has both good aspects (e.g. faster boot times and removal of the nasty nest of shell scripts) as well as bad ones (incredibly monolithic). There are arguments on both sides of the fence. It's just that you're more likely to hear from people who dislike something than you are to hear from people that like it. (And at this point most of the systemd proponents have given up talking about it because of the incredibly vocal and relentless opposition.)