Some people think it handles too many low-level systems. It's a valid concern because if systemd itself were to become compromised (like Xz Utils was) or a serious bug was introduced, all of the userland processes would be affected. People who are stuck in the 90s and think that the Unix philosophy is still relevant will also point out that it's a needlessly complex software suite and we should all go back to writing initscripts in bash.

Red Hat, the owner of systemd, has also had its fair share of controversies. It's a company that many distrust.

Ultimately, those whose opinion mattered the most decided that systemd's benefits outweigh the risks and drawbacks. Debian held a vote to determine the project's future regarding init systems. Arch Linux replaced initscripts because systemd was simply better, and replicating and maintaining its features (like starting services once their dependencies are running) with initscripts would've been unjustifiably complicated.