Trainers are aerobatcially challenged for a number of reasons:
1. Control surfaces are smaller (with less throw) - makes aerobatics slower.
2. Wings have dihedral, washout, and are often flat-bottomed - makes inverted flight far more aerodynamically difficult because a lot of elevator is required to maintain it, and also, because of the washout and wing-shape, the stall speed inverted is often much higher than right-way-up.

Given that most aerobatic manouvers require consistency and smoothness, it is always going to be more difficult to perform the manouvers with a trainer wich is VERY biased toward flying right-side-up. Further, many manouvers require the plane to maintain knife-edge travel which requires a large rudder. Things like slow rolls, 4 point rolls, etc all require Knife-edge capabilities. Many trainers simply do not have the rudder "authority" to do them.

Finally, aerobatic planes have a very good seperation between the pitch, yaw, and roll axi. The rudder affects yaw only. The elevator affects pitch only, and the ailerons affect roll only. Trainers have dihedral wings, and the controls often affect more than one axis (rudder affects yaw and roll axis on a trainer). Thus, to perform aerobatics, you have to reverse the "unwanted" axis rotations.... for example, in a hammer-head, you have to use throttle, rudder, and opposite aileron to get a straight hammer-head with a trainer, but with a more aerobatic plane, you could do it with pretty much rudder and throttle only.

So, some moves are simply not possible with a trainer because the control surfaces do not have enought authority (like a knife-edge), and other moves are more complicated by having to counter-act the adverse axis change with the trainer-s control surfaces.

Bottom line, there is a lot a trainer CAN do, and if you can do the moves with a trainer, you can be pretty confident that you will be able to master them on a an arerobatic plane as well (aerobatics becomes easier with an aerobatic plane!).

gus