There are some really great etudes out there that have a few things in common, besides being hard. The main thing is they have traps. These take many forms and actually most of the problems with working out these etudes are related to the following traps.
1. Breathing. One classic trap in Kopprasch and Gallay is phrase length; they are geared to being phrases you can make but only with great breathing and the right tempo. Maxime-Alphonse gets even meaner; there are phrases that simply can’t be done and then you have to figure out how to make bad breaths sound good.
2. Dynamics and contrasts. A lot of the classic etudes are very black and white. For example in Kopprasch many phrases are either soft or loud, short, long, etc. If you play it in the lesson all at mezzo-something with a medium articulation you are giving the teacher much to comment upon!
3. Terms. One of my favorites to see in an etude is L’istesso tempo. If you make a tempo change, it is a great chance for the teacher to ask, “so, what does L’istesso tempo mean?” You should know this from the preparation of the etude. If you answer “streetcar” it does not get you extra points. You should know every term in every etude and excerpt before you bring it in to your teachers; this is basic preparation.
4. Range. Besides high range I love etudes that run you down to the bottom of your range. It can be a huge challenge to get the volume and articulations to work in the Neuling low horn etudes for example but that is what it takes to really have the range worked out–and it must be worked out to achieve a high level on the horn.
5. Nuance. For this Maxime-Alphonse is great. He expected quite a variety of nuance of articulations from very long to very short at a variety of dynamics, something that must be in total control to play musically.
6. Musicality. For this there are some great etudes, especially in Maxime-Alphonse and Gallay. Some of their etudes are written of course more to learn to bang out the notes, but there are others that are true musical challenges such as the Gallay Unmeasured Preludes.
7. Transpositions. The printed transpositions in Kopprasch actually are not original to their first published version but there is a point that you just have to learn how to transpose well and etudes are the starting point for this, especially Kopprasch and Gallay.
8. Scoping out your problems. Finally, the goal is not so much to be mean I think from any of these teachers that wrote etudes, the point is to scope out your problems and point out what you need to work on. To reach the next level you always need to keep pushing past the things that hold you back
The reason I love these etudes above of Kopprasch, Gallay, Maxime-Alphonse, and Neuling is not because they are full of traps but because they are effective for working on problems and making technical progress. And I also don’t think these teachers actually were mean; they most certainly wanted to guide their students effectively toward better technique, the goal of any good teacher.