Gunner is not easier to learn or teach. You have to explain three different ammos, ranges, lead, order of firing, timing, gun maintenance, emergency repairs, spots, AND they have to talk because gunner/pilot feedback is important.
Engineers? "If it is red, hit it with a spanner. If it is not, hit it with a mallet. Use a gun when I tell you. Use your special ammo when they are close/far."
And I will agree on no novice pilots as well outside of custom matches.