Regarding piano technique, I have learned firsthand that Hanon and various books solely on technique are practically useless: in fact, from my point of view, they are harmful because they stem from a reasoning process that the intuitive method performs. Let me explain: I know that for some this might be obvious, but I only knew 20% of what needs to be done in a practice session (even though I studied very little as a teenager). (I learned this by reading a book that deals with piano technique and musical study: if you want, I will give you the name and the link with some parts in downloadable free PDFs from the edition prior to mine. I am not citing the link because I don't know if doing so would introduce spam, since it is a site intended for selling a book).
Many students use a more or less standard procedure such as:
1 - Studying scales mechanically and Hanon-type exercises
2 - Picking up a new piece, reading it slowly hands together while trying to increase the execution speed
3 - At the end of the practice session, playing at sustained speeds to enjoy the piece
4 - Once successful, memorizing it and more or less mastering it
Without claiming to be a prodigy, if someone, for example, studies Bach's Two-Part Inventions, they will find themselves experiencing mental setbacks because the method does not teach how, first of all, to reconcile the two hands playing distinct parts that create seemingly insurmountable walls.
Technique is only a means: it must be acquired through music. Quoting Bach, in my opinion, in one of the phrases that struck me most for its deep meaning: "All musical instruments are easy to play... you just have to press the right key at the right time, and the instrument will play itself."
To me, it made me understand that music, before being a series of emotions, is a combination of sounds with more or less nuances within a unit of time: if one wants to make music, one must learn how to bring it out by slowing down, performing and repeating or breaking certain passages, and digging into oneself step by step, cultivating a study method that leads to a goal. If one proceeds by rationally analyzing what they want to do, then they will be able to make music by acquiring the technique accordingly. If one uses an intuitive, little-rational study method, it is very unlikely they will manage to play decently.
This is my opinion from the height of my incompetence! 😆