i've been considering this for overwintering, to be causing problems...i don't really know but i can tell you something.
look closely for chalk brood because what i have observed is...
i had a bit weak hive a month ago, and i fed it, a little, the hive has an open feeder within the hive (it's different type of hive to what you're used to) so...the next day checked and they didn't take all the syrup, and in the next week, the hive head somwhat severe chalk brood. i assumed this is due to increased moisture. now...with boardman feeder you actually shouldn't have this problem, since the water isn't actually exposed, and it took me the whole post to realise this and to realise my writing was in VAIN. ahahah LOL, impressed by my own stupidity.
i can't see how it could cause problems, but bees do like minerals in their water so a tiny pinch of salt might encourage them, don't know about other minerals though...