I think the point of 'easy' or 'hard' was in relation to a newb trying to learn how to fly and get into this game.
Thinking for a newb, the Spits are the 'easiest' to learn because, IMO, they help the pilot in many aspects of flight and combat from the getgo than do some other planes. The best word for the Spits I can think of is its 'stable', and this is a big help to a new pilot. More 'unstable' planes like the German iron are harder to learn to fly well, but their inherit instability, if managed well makes them capabile of exceptional maneuvers.
All this means, in a Spit, the newb doesn't need think about, manage or master too many things right off the bat to be able to fly passibly well, which in turn means he may stick with it longer and learn more ACM over time.
What makes Spits 'easy' to learn in-
1. The Spits feature several models with various combinations of speed, turn rate, climb rate, etc. so they matchup favorably with most planes.
2. They have the best trajectory 20mm cannons in the game so shooting might be a little easier to learn in them.
3. They retain and recover their energy very well.
4. They are a very stable gun platform.
5. Very responsive nose (rudder control) for gun shots.
6. No bad stall characteristics, and good stall recovery capability.
7. They don't have compressions issues.
8. Good cockpit visibility.
9. Good at all altitudes (XIV excepted)