There could be many reasons why an evaluation board is not fit to be used in a final product.
1) An eval board is expected to be used at lab temperatures. A product would typically be expected to work over a significant temperature range.
2) An eval board is expected to be used under the supervision of an experienced engineer, and so crashes and hangs may not necessarily render it useless. That's not the case for a product.
3) An eval board need not meet RoHS guidelines on nasty chemicals used in manufacture, lead-bearing solder is a typical infringement, or RFI emissions or susceptibility. They typically get dispensation on the basis that they are not for resale, and very few will be made. Product cannot be sold with these problems.