You can be guaranteed that it'll be dark, cold, wet, windy, icey, snowy and hard going for at least some of it. Possibly all
However it'll be quiet, the views on a clear winter day are properly stunning and you may well have one of the most memorable experiences of your life.
However you do need to be very fit and confident in winter weather. The track is waymarked but in a whiteout that's as good as useless. Your navigation skills will need to be on point and you should consider things like crampons and an ice axe for sections like the devil's staircase.
You don't say what your Hill experience is so it's difficult to make a good recommendation for you. If you're a summer walker with zero winter skills then I'd be inclined to advise against it.
For emergencies you should take a mobile phone (999 and ask for mountain rescue) and I'd also recommend taking plenty of spare charging packs and using something like Viewranger as a backup to a paper map. Leave a detailed itinerary with someone and advise your accommodation provider (if you're not wild camping) of your ETA.
The biggest challenge is staying dry and warm. I'm usually ok camping out for a few days at a time in winter but that's with plenty changes of clothes and socks. Camping is an option but you'd probably be best booking at least a few nights in B&Bs. Nothing worse than being cold and wet to thoroughly sap your energy and morale.
You'll be lucky to get more than 6 hours of daylight in December so do keep that in mind when you're tackling the more technical sections from Rannoch Moor onwards.
Apologies if that's a little negative! It could be an amazing trip but only if you have the confidence and skills to do it.