I had the chance to see a loading and firing demonstration of the Ferguson at the Rev. War reenactment in Camden this past November. The reenacter listed several disadvantages of the Ferguson. One was the maintenance. I can't remember the exact figure, but it was something along the lines of 500 gallons of hot water needed to clean all the rifles Ferguson had wanted to arm his rifle company. This would have been a logistical nightmare for an 18th century army. Another was the stock had a tendency to crack where the action screwed through the stock, since so much wood was removed from there. Another was the cost. At the time, a Brown Bess cost the British government 4 pounds to build--Fergusons cost 8 pounds each, and one maker had sued the British government for more money before his rifles were delivered. And another disadvantage was what you mentioned--the action had a tendency to lock up after the second shot. To overcome this, the riflemen had to lube the action with tallow before firing. This apparently reduced fouling. It was said that the evening before the Battle of Brandywine, Ferguson was going through the British camps searching for tallow. Hope this answers your question.
