Well, the problem is, you’re flying along at 150-200 knots, several hundred feet above the ground. That means your target is traveling, relative to you, 150-200 knots several hundred feet away. Bullet drop becomes an issue at the distances and speeds involved, making it hard to actually hit what you are trying to hit.
OTOH, you could ditch the cannon, load up with more .50 cal machine guns, and just perforate your target with lots and lots of (relatively) smaller bullets that are easier to actually paint your target with.
From what I see, the problems with using the Gatling against European professional armies had less to do with the infantry tactics than the opposing artillery. A Gatling emplacement was pretty much immobile and high-profile, making it a sitting duck against long-range artillery attacks. Tactical doctrine in the early 20th century was to soften up enemy positions with artillery fire prior to sending in waves of infantry to occupy and control the position. The cannons of the time could easily destroy your Gatling guns from far outside the Gatlings’ range. Once that happens a few times, you learn your lesson and start bringing something else to the party. The man-portable machine guns could be kept tucked away in the bunker until the enemy infantry started their assault, at which point you set 'em up & started laying down the hurt.
The weapon they are talking about is specifically the Model 1862 Gatling gun. This weapon used a gravity-fed box magazine on top and required someone to turn the crank with their hand. Every time the box magazine ran out, the gun would stop while it was reloaded.
At the time, belt-fed automatics such as the Browning and the Maxim water-cooled guns were in fact superior to the M1862 gatling. They were smaller, lighter, and used belts that fired longer. As technology improved, gatling guns would make a comeback (as many users have already pointed out).
The point is that they were talking about the M1862 in particular, and were not talking about gatling gun technology in general.
Gatlings don’t make much sense as an infantry weapon, compared to single-barrel machine guns. Even by WWI, machine guns could fire as quickly as was desireable (putting more than two bullets in any target is just wasting ammo, and a human can only swing the gun so fast. There’s definitely a optimum rate of fire for infantry machine guns). Gatlings do have an advantage in that over the medium-term (say, 10 minutes of heavy firing), the barrel of a machine gun will get too hot to keep firing. A gatling spreads the heat over six barrels so it can fire at least six times as long before overheating. In fact, I think this is the big advantage of gatlings for aircraft.
But they solved that issue for infantry machine guns by making the barrels removable, and giving the gunner an extra barrel or two to carry. When the barrel got too hot, the gunner just swapped in another one. A machine gun plus an extra barrel is much lighter and more portable than a gatling gun.
