For me, I guess several reasons:
* Log files aren't where I expect them. I can't just tail the right log file, I have to figure out a load of options to journalctl instead. Its defaults are annoyingly bad and I usually end up having to type long things to limit the range to something useful.
* The journal grows massively and is unbounded by default. Many times I set up a machine, and then it runs out of disk space. It's now instinctive for me to now check whether it's /var/log/journal that's using it all. In fact, I just double checked on the machine I happened to be using now, and the journal was 2.2GB.
* It's terribly documented, or at least not in the way that's familiar to older UNIX folks. It took me about 30 minutes of googling to figure out how to change the name recorded in the journal, which defaults to the command name in ExecStart (and so was really usefully just unshare in multiple of my services). For anyone that's wondering it's SyslogIdentifier - good luck finding that yourself. It makes sense, but it's woefully under documented anywhere.
* Whenever you change files that used to be the end of it, e.g. /etc/fstab, now you have to remember to `sysctl restart systemd-mount` - why can't whatever needs it just watch the file for changes instead?
* Too many things just happen in the background that never used to. Just now for instance, I manually unmounted a drive to resize2fs it because I wanted to move the underlying data. Between running e2fsck and resize2fs, systemd had already re-mounted it read/write. Luckily, resize2fs is smart enough to tell you. If I'd been doing the actual task using dd to copy the data elsewhere, I'd have ended up with a corrupted copy.
* Just yesterday, I discovered that edits under /etc/network/interfaces.d were just silently ignored, and I had to learn the new systemd way of doing it. I never did figure out how to set the MTU in the configuration either.
* The configuration files feels Windows-like and not UNIX-like
That said, I've reluctantly started creating systemd services instead of rolling my own init scripts, and it's quite nice not having to copy all the boilerplate from elsewhere and just having a handful of lines of config. But most of the time, I feel like I'm fighting systemd rather than working with it.