A 'good' trainer is one that is effective with a variety of horses. A good trainer can teach horses with the innate ability to do something to do this to the best of the horse's ability without resentment or resistance.
This means that the trainer must not only produce acceptable to very good results, but must adjust the method of teaching to fit that individual horse to bring out the best results that the horse is able to do. If the horse is free of resentment, fear or anxiety, then the trainer has good training ability.
It is all about communication. Not every trainer can communicate with all kinds of horses. Some horses are a lot easier to communicate with. Some trainers adjust better to different kinds of horses. Results count a great deal, but are not the only consideration. If the trainer could get past any resistance on the road to those good results with little or no fear, anxiety or harshness, then the trainer has been effective and the methods used on that particular horse have been the right ones for that horse.
Methods can only be taught to some degree. Any method requires good timing and feel and very good judgement skills. These skills are very difficult to teach even when working one on one with a very dedicated student. Add to that -- methods need to differ from horse to horse. Methods used to teach timid, frightful horses need to be a lot different than methods used to teach willful, pushy horses, Even different methods are needed to teach spoiled horses.