I will say this: My first course was with American School of Piano Tuning when I was a 17 year old teenager. What was taught were the very basics. The tools were adequate for that but the tuning hammer (at the time) could have been better. That was 1969. A new student does not need to have a very expensive tuning hammer. There are choices which can be made for a perfectly adequate one that does not cost greatly.
What I have the most to criticize are the tuning instructions. Those could be improved. I struggled greatly to rise above what I was initially taught to rise to the level of RPT. It took attending the first PTG convention that I attended in 1979 to have my eyes and ears opened.
After a few years of hard practice, totally disregarding what I had learned from the American school, I was able to qualify at a superior level as an RPT. The American school is chosen by many. I understand that but it should be considered to be only as an introduction to piano technology, not as everything a technician needs to know.
In summation, the American School has helped countless people to have an introduction to Piano Technology. For some, who were merely curious or only wanted to experiment with piano tuning and repairs as a hobby, it has been useful and adequate. For others who genuinely sought a career as a piano technician, it was readily understood that there was far more to the profession than what is taught in that course.
In that respect, it is useful. One may determine from the material and tools whether one may want to go further. I can actually say that I am glad to have found the American School when I did. It lead to my career as a Piano Technician.