1. Not all companies offer DRIPs, only some of the large public companies offer it.
2. Restrictions usually apply, such as the amount you are permitted to invest.
3. Many DRIPs start charging fees, losing its cost advantage over discount brokers.
4. Time consuming, for both transaction (many DRIPs require you to execute via U.S. mail, and only do it a few times a month) and record keeping (especially when it comes to tax filing time).