Price
The first disadvantage of frost-free freezers is the price. They typically cost more than a conventional freezer that must be defrosted. In addition, the extra cost may force you to purchase a smaller freezer than you might originally have intended to purchase, or lead you to have to sacrifice in other areas. The ongoing electrical requirements of the frost-free unit will also be a consideration, since it will cost you more per month in utilities to operate. If price is a concern, the traditional freezer will likely be your better choice.
Drying Out Food
Another major disadvantage of frost-free units is that they tend to dry out food, which leads to food spoilage. The cycling of the frost-free units seems to be more conducive to freezer burn in foods. This can be an annoyance, and a monetary consideration, since meats and frozen goods are typically more expensive. So unless you can use the food in your freezer rapidly enough to avoid the drying issues, a frost-free freezer is probably not the best choice.
Waste
One final disadvantage of the frost-free freezer is that its very nature and simplicity of use leads to a degree of selective memory as to what is actually in the freezer. When defrosting a conventional freezer, you have to remove each package before the freezer can be defrosted, so you have an excellent opportunity to look at each item and check expiration dates, condition of the packaging, and other factors, as well as to rotate foods which are close to expiring. When you do not have to do this because you have a frost-free unit, you may forget what is there, and eventually the food will spoil, and you will lose it after all.