Regarded as bad, bad, bad. It contains all sorts of other things which will give the bees a funny tummy. Even brown sugar is to be avoided. So pure sugar only. That is for winter feeding when the bees are indoors for long periods. I would not risk it even in the summer. But someone might enlighten me otherwise.
This was for winter feeding and spring feeding. The concept was you put the fondant over the holes in the crown board and because of the consistency, the bees would only feed when they had to rather than take it all down into the super thus resulting in a possible dilution of the honey with excess syrup come spring.