I think the issue is just stability in general. I just moved to a new server recently, and I decided to go with CentOS 6 instead of 7, as people were reporting that CentOS 7 is just a little less stable in general than 6. (Although systemd probably isn't entirely responsible for the lower stability in CentOS 7, it probably is one of the major changes in CentOS 7 so it likely takes a good share of the blame).

You always end up with bugs in released software, even CentOS. Some bugs only show up in certain situations, and might be difficult to track down. The only way to give a good guarantee of stability is for a large number of people to use the damn thing for years on end!