I can't find any reason that you explicitly do not ever want to use corn syrup. The sugars involved in the substitution (dextrose in the case of corn sugar, and fructose in the case of corn syrup) are very similar, and both are fermentable. However, there are some pitfalls to avoid, and corn sugar doesn't come with the same disadvantages. First, corn syrup can have a lot of additives, including preservatives. Any preservative will inhibit if not kill your yeast, so look for a brand with none. Other additives include vanilla, molasses, salt, and perhaps some others. None of those latter few will do any real harm to your beer, but you may experience some off flavors as a result. Lastly, corn syrup has water added to the sugar(s), so measuring it accurately as a substitute may not be as easy as it first appears. As a starting point, I'd guess using 25% more corn syrup (by weight) than corn sugar.
Perhaps more of an answer than you wanted...