Missing skill build-up lesson chain
The lessons are too easy to pass, and also they are pretty uncomplicated compared to the skill tests, which suddenly require a sudden change in speed in reading and changing chords, that can not be acquired with doing the lessons, since those are usually of low speed. This easily builds up an illusion of progress in the user, which turns into acrid disappointment and frustration when missing the skill test due to inadequate skills, but all-golds in the lessons. There should be a better balance.
My suggestion would be a larger number of lessons in between levels, which first introduce the new material, then focus on building up the users agility in using them. A possibility for the user to set the minimum pass criteria (like % of allowed note/chord misses, % of timing errors, etc.) to a higher level might also help to avoid the illusion of progress.
I've always found the skill tests to be very appropriate for my ability after doing well on the previous lessons.