Excess of food in whatever form can clog the brood nest.
Think from the bees' perspective, as I understand it:
Nectar is a sugar solution which they concentrate down to honey by dehydration. Honey is useful for stores as it's water content is low enough to inhibit mould formation.
The dehydration of nectar/sugar solution uses energy. The more dilute it is to start with, the more energy it will take to concentrate.
Fondant does not need concentrating down.
In winter you want them to conserve energy so it's more common to feed more concentrated sources of sugar such as fondant if store levels are low (and if you're worried they might be low, just feed them, better too much than starvation!).
Some (I believe
@Finman) will dunk a frame in strong syrup so the cells fill up with it and put this inside the hive for the bees as an alternative to fondant. I have not tried this myself.
Nectar flows also stimulate the queen to lay to increase the colony size to exploit the nectar.
You don't want this in winter as you will end up with more bees and brood which will then consume more stores. Which means you have to pay for something to feed them.
You do want this in early spring (or prior to an anticipated nectar flow) so you can increase colony size ready for the nectar flow. So dilute syrup tends to be fed in spring. At this time you may be doing weekly inspections so can adjust your feeding rates or switch out frames if you think you're losing brood space.
Concentrated syrup does not have the same effect on stimulating laying as dilute syrup and takes less energy to dehydrate. This is why it is often fed in Autumn to boost stores pre-winter. Fondant can be used as an alternative.
Some use fondant in mating nucs too but that's probably outside the immediate sphere of the question.
Bees will also be disinclined to take down syrup in winter.