Yeah, in-place upgrades between CentOS releases are not really supported, just as they are not really supported with RHEL.
However, this also comes with a perk. CentOS releases are generally the "current" release for at least three years, as opposed to two with Debian. Also, major releases are supported with critical security updates for years after they are no longer current. For example, CentOS 6 will still receive patches until November 2020.
Of course, whether this is important to you will vary based upon your needs. For some, the 2 year releases with an easy upgrade path between major releases will be the more important factor.
Having dealt with in the past RHEL 6 to 7 is supported but only for the server edition: https://access.redhat.com/solutions/637583. At a certain point I'm not sure what Debian does so much better, but that's just because I have not really needed to investigate it to the same depth as RHEL.
It will probably work for 7 -> 8, the upgrade tool is a recent addition. It already worked in RHEL for 6 -> 7.
That being said, for RHEL 6 -> 7 I did a clean reinstall due to the large architectural changes and it was the right decision.
AFAIK CentOS does not support distro-upgrades (eg. upgrading from CentOS 6 to CentOS 7)[1], we always end up making a fresh install whenever we want to upgrade, i've tried to use the UpgradeTool a few times but it didn't work (there was always something blocking the upgrade).
I believe this would be a point for Debian, the upgrade between releases just work :)
[1]https://wiki.centos.org/TipsAndTricks/CentOSUpgradeTool