A couple things about the Ferguson rifle...
Even with cleaner burning modern black powder, the thing fouls up past use in about 6 shots. This was a little worse than the muskets of the day. The muskets oft had special loads with smaller balls set up for when several shots had been fired and the fouling was building up.
That was a small drawback, the better accuracy and speed made the Ferguson a good battle arm, especailly when placed in the hands of someone who could really shoot.
And that's probably why it was withdrawn from use. Politics played a part, Major Ferguson had stepped on some toes and the Brits were losing lots of officers to 2nd generation Scot-Americans in the Continental Army with rifles and grudges against the British from the Clearances that killed off 1/3 of the Scots and caused another 1/3 to emigrate, mostly to America follwing the battle of Culloden, 1746.
The Brits were loath to use riflemen to take out enemy officers because of the class thing. Officers were nobility, and a weapon that effective against single targets wasn't what they wanted around.
They got over this by the time of the Napoleonic Wars, and troops equipped with the Baker Rifle were used to take out officers and artillery crews.
The big thing about the Ferguson and Hall designs was it showed that breech loading, with or without cartridges, was feasible, and led to the Sharps and Snyder precussion arms and thence to cartridge arms.