In my experience, plastic insert feeders that fit inside medium or shallow supers are dangerous because they don’t provide the bees convenient access to the syrup. Using an insert feeder to build up a nuc could be disastrous, especially in a cold climate like Newfoundland.
I bought an insert feeder during my second spring of beekeeping in 2011 because it seemed like a cheaper alternative to a hive top feeder. But I could never get the bees to take syrup from the feeder. (I’ve heard the same from numerous beekeepers over the past four years.) My bees would have starved had I kept trying to feed them with the insert feeder.