I am starting to feel that ZLM is too much problems to administrating. I
only uses it for patching and when I got the following information I thought
enough is enough.

ZLM is designed so that the ZLM server is the content delivery source for
the managed workstations. To avoid software install issue, we designed the
ZLM agent to remove all other software source on the managed device. You
should, if possible, provide all additional software with bundles through
ZLM once that (automated?) install process is finished and the agent got
installed. The agent does not stop you from re-adding other software sources
after being installed but you might experience issue which are not easy to
solve, when providing software thorugh different channels to the device.

The other alternative is to use Subscription Management Tool,

Is there anyone who has tried patching SLES and OES with that product? Is
there any reasons for continue to use ZLM for patching purposes?