I suppose whatever you learn on has its own little foibles which need to be accounted for.
I learned on the Gazelle and the Sea King which, from a purely poling aspect are fairly easy to fly. Military machines, I think, need to allow the pilot a certain degree of spare capacity to 'fight' the machine, ie carry out its role without crashing.
Something like the R22, I find requires more of the pilots brain power to merely fly the aircraft and does not allow much scope for 'hands off' flying in order to carry out cockpit management related tasks. It's also so underpowered (esp in the summer) that a permanent carb heat would probably not do. So that's something else you just have to keep your eye on.
A good training machine must be able to allow the student to make some mistakes without dying and allow him or her the spare capacity to actually learn something rather than be permanently struggling with an angry beast.