As well as greater life to start with, many pieces of down equipment can be restored with a new filling, either by the original manufacturer or other down specialist. This is usually much cheaper than a new garment, but is generally not economic for synthetic insulations as they're usually sewn into the equipment rather than left loose. It's not all good though. Synthetic insulation performs much better when it's wet, so if there's a very good chance that your equipment is in for a soaking, down is not so good. Just because they don't degrade as much as down when wet doesn't mean they don't degrade at all.
Performance drops, and weight increases.
Whatever insulation you use, you're better off keeping it totally dry. I use down bags in Scotland, camping, bivvying and bothying, which is not known for its aridity, and don't have a problem keeping it dry. In really cold climates, vapour coming off your body can condense in the bag itself, dampening the insulation, and this will affect down more than synthetic fillings. Not really an issue in Scotland, though I often wish it was. Next problem with down is it's less effective than synthetic insulation when compressed, like under your body for example.