One of the big issues a lot of people have (or at least had, back when it was first becoming a thing) was that systemd uses binary log files rather than traditional text ones, which makes them harder to deal with (e.g. you can't `tail` or `grep` them, at least not directly). Another common gripe is that while it started out as an init system, a rather large portion of userland on a typical system depends on one or more systemd components directly or indirectly, making it somewhat difficult to avoid it if you'd prefer not to use it.

My biggest issue with systemd is that when something goes wrong, it's pretty impossible to tell why. As an example, virtually every system I've used with systemd after a few months starts to have different services fail to stop on shutdown, causing a timeout of (by default) one-and-a-half minutes. From searches that I've done online, I don't seem to be the only person who runs into this, but I haven't found any good solution, leaving me with the options to reduce the timeout to something more manageable, force shutdown my laptop literally every time I'm done using it, or sit through 90 seconds of systemd trying and failing to stop whichever service is failing that time. Maybe I'm just lucky, but it's fairly rare that any of my non-systemd systems is unable to shut down properly once, let alone every single time consistently.