For Cairngorms in December, the limiting factor may be water levels in the rivers, rather than ice and snow. December is traditionally a ‘changeable’ month in weather terms, so you could get cold and dry, cold and wet, or just very wet indeed with rain and snowmelt combining to make river crossings like the Tilt, Geldie or Feshie potentially lethal.
However, knowing where the bothies and mountain shelters are and with a flexible attitude to route plans A, B & C throughout, you can do this reasonably safely. Comms will obviously be an issue in the extended remote sections and daylight too, with no better than 7 hours of daylight on a bright day and less if it’s minging. Local knowledge will get you details of bothie locations, where to expect phone signals to return and the locations of remote but occupied cottages.
If the weather turns really sour on you at the start of this sort of winter trip, it is possible to pull together some properly large loops around say the Aviemore or Braemar trails, without crossing major rivers and staying within reasonable striking distance of both civilisation and phone signals. There are bothies with camping space alongside in both Feshie and in Rothiemurchus, so I’d suggest if you are going to plan a winter Cairngorm loop, start and finish at Aviemore or Newtonmore. This gives you the bad weather option of staying local but still getting a good thrash around and kit test.