Any zwave switch which directly controls the current load to the fitting should continue to perform that function even if the home automation system is not operating. I’m not aware of any models that don’t do this.
The “scene controller” or “button controller” functionality which operates by sending a message to the hub will not be available if the home automation system is not operating.
(There is another “scene controller” feature which operates by having the switch send its message directly to another Z wave device in the same room, and that would also work even if the hub or not functioning, but that modality is not normally used with SmartThings.)
So it just depends on the exact device that is being used and how it was set up. Many zwave 3 wire switches do control the load to that specific fitting and so that would work exactly as the In-wall relay does.