Breakfalls can result in severe dizzyness, and I'm almost sure could eventually cause Parkinsons disease, if done exclusively! Always try to roll instead as it provides less shock to the brain. If you are thrown too fast to roll you will realise because your back hurtles towards the floor directly and no rotation of the body is possible. In these cases your arm should automatically break-fall. If not, try doing a few rolls where you gently slap the mat during the roll.
I would be careful about being 'sensetive' as uke. Aikido requires non-competitiveness. An instructor I know said Ueshiba spoke of the uke being the devil and the nage the angel. i.e. the uke has a responsbility to give an aggressive attack whilst the nage should be sensetive and blend. If uke blends with nage too much it can weaken the attack or produce a counter technique. Usually this is not the purpose of the training method.