I think he has a point. The gunner role is the easiest to learn and has the lowest skillcap. He summarizes the experience pretty accurately, all you do is sit on your main gun and shoot.
Of course there are different guns for different purposes but you learn quickly not to use the gatling for destroying the balloon. On some ships you even have to change weapon depending on where the enemy is (Galleon) but there is not much more to gunning.
Okay, even while I seem biased, I have to say being a good gunner is in no way any easier than being a good Engineer. Engineer usually has more tools to play with, but as a gunner you got half, if not two thirds, of the ship's damage output on your shoulders. If you miss with a Hwacha, it is no components down, if you miss with a gatling too much, it is an enemy hull armor up. The basic concept is very simple, point-and-shoot, but taking into account the x,y,z and their differences when accounting for two moving ships still befuddles me sometimes.
Still, I admit I am rather a gunner with one focused mission than an engineer with various duties.