The major risk is not the deco portion of the dive. Most technical divers configure and do the gas planning to allow for the loss of any single deco gas and have contingency plans to do the longer deco off a single remaining deco gas or on back gas. They also do contingency plans for longer deco in the event they are delayed in leaving the bottom. In other words they plan and configure to be fully independent anyway and the support available from a team is just icing on the cake.
The major risk diving 15 miles off shore (deco or not) is the risk imposed by the unattended boat. If the anchor comes loose or the anchor line separates, you are now 15 miles off shore with no boat. In some areas the risks may also include the unattended boat getting stolen, getting run down by a freighter, getting swamped during a sudden squall, etc, etc, etc.
In my opinion diving offshore with an unattended boat any farther off shore than you can easily swim back to shore is really, really stupid. The availability of diver friendly EPRBs would help but makes this practice only slightly less stupid.