I've been working with UNC paths, off and on, for more than 25 years.
At their best, they are convenient.
At their worst, they are extremely confusing. Backslashes are escape characters in many languages and applications. UNC paths sometimes have associated authentication (username and password), which you need to specify. There are "workgroup" and domain considerations. There are special ways of formatting UNC paths to include some of the above information in the path itself.
I now avoid UNC paths for some of the reasons above. I like simple. I like reliable.
With all of that said: I recommend that you map your UNC path to an unused drive letter. Then all the above issues will disappear. MC will work with it transparently. As a bonus, if you ever change servers, IP addresses, or paths on the server, your drive letter can remain exactly the same. You just map to a new location.
If that's not acceptable to you and you feel the need to use UNC paths then I wish you good luck. Actually I wish you good luck either way.
Brian.