Can I compare a single value to a multiselect list in a workflow condition? For example: If Employee (list field) is any of cust_field_a (multi-select field) then xyz..

I struggled with this too, I needed to do something similar. In the end, I just split the multiselect value into 3 separate employee fields and worked my WF based on those 3 fields. I guess this advice is only helpful if you know you will have a hard limit (of 3 in my case)

Yeah, I was never able to figure out a way to do this in workflow either.

I was able to figure it out with an INST formula:

TO_CHAR(INSTR({employee.custentity_employee.id},{user.id})) != '0'

aaaaaaaaaaaaaaaaaaaaaaaaaaand that goes into the knowledge banks for future use!