Migrate d2d UI Comment Fields
There is no bug.  I ran the import the first time, then discovered that I was migrating nodes to the wrong content type, so ran it again with the correct content type.

Thing is, if you don't configure your destination content type as allowing comments, then comment migration will be skipped for all sources mapped to that destination.

So, I thought d2d ui wasn't working correctly when all the time it was my configuration.  Oops!