Problems can include getting a finish that lasts. Generally a DIY painted kitchen wears incredibly quickly. When you are painting on top of an already used cabinet, you need to spend a lot of time cleaning, sanding and then sanding and cleaning again, all before the first layer of your new paint colour goes down. The entire process is very labour intensive. Even if your cabinets appears clean, there will be layers of grease accumulated that has built up over time. It is very difficult to actually remove all of these layers to a professional looking standard. This becomes even more difficult if your kitchen cabinet has beading.
Repainting a kitchen can provide a short term cosmetic update, like repainting a car. But it is not fixing the real issue, this may be the layout of the kitchen, your doors have dropped and no longer line up next to each other, worktops scratch and dent, or the carcass to the cabinet has blown.