Downtime and hotpatch modes should only be used when instructed.
You should apply patches on a test system the same way as you would do on a production system to avoid any unexpected failures.
12.2 - How To Use Online Patching Related To "Hotpatch" Mode, And Is It Possible and/or Supported To Apply Or Abort A Patch Applied To The Run Filesystem Via "Hotpatch" (Instead Of Using ADOP) (Doc ID 2054074.1)
Downtime mode is much safer than hotpatch mode simply because services are down.
There should be no issue with running a patch in downtime mode in a non-production system. The only risk is that unless a patch is listed as runnable in downtime mode then it hasnt been tested in that fashion by Oracle.
Hence why we say for non-production systems only unless specified.
Again, saying this there should be no reason why it would fail. If it does then Oracle would certainly assist with fixing any issue, as long as its non-production