My experience:
You can use it for the Hosted Engine VM and the Hosts. Be patient, don't interrupt the yum update process, and follow the instructions.
If you have a locale different than US-UTF8, please change it for US-UTF8 before the upgrade process. Where? /etc/locale.conf on CentOS. If you are using e.g. puppet, please deactivate it on upgrade process, to avoid the locale change(if you have something on puppet that changes it).
Problems:
I still having problems with glusterfs(Peer Rejected) and unstable, but it probably happens because I copied the UUID from the broke server from glusterfs and added the new installed server with same ip back to gluster.
IMHO:
Please check that you have the engine backup done. Save it somewhere, NFS, rsync-it to another server....
When running engine-setup after the yum update on ovirt-engine VM: don't to the in place upgrade from postgresql. It's really nice to have it, but you you can avoid risks, why do so?
Keep the Backup from Postgresql.
Previous Version: 4.1.x
Updated to: 4.2
Setup:
CentOS 7.4.108
4 Servers, 3 with Gluster for engine.
If you have questions....
Best Regards,
Gabriel