Feature creep and a refusal to take responsibility for or fix bugs. But I personally I stopped using it because my RAID on LUKS set up with 4 drives had a 3 in 4 chance of failing to boot (after hanging for 6+ minutes waiting for timers to run out), and shutting down would take 6+ minutes due to retarded timers. I was able to fix the long timers issue, but not the boot time Russian roulette, and since this was a remote system this shit was not acceptable. The only solution I could find was to add a timer to the rescue shell prompt so if no keys were pressed it would just keep rebooting until it worked. The bug was known but not fixed for over a year. It might still exist, idk. Point being no matter how complicated or unusual the disk setup, I never had issues like this using SysV or RC scripts, and if I did I would have been able to fix them pretty easily on my own.
SystemD is probably fine for most users. I don't mind it on most of my laptops and SoCs, but I'll never use it on a server or desktop again.