This had a core cannon of six books from the Middle Ages and one post Renaissance book-the Mesilat Yesharim.The complaints about this movement were varied. The Chazon Ish asked "What good is Musar if you do not know Halacha?"
Clearly this is a good point but I have a further complaint. What good is Musar if you do not know the philosophy behind the books? What I think is important about Musar only comes because of the general paradigm of Musar i.e. it imbibes into tone the general world view of Torah.
Also I want to mention a few more flaws in the movement that should be taken constructive criticism not destructive criticism. Things that should be corrected to make the movement better than it already is. First get rid of all the moral busybodies
Second the are issues that come up in Musar that are not dealt with. The Ramchal emphasized learning Kabalah. The Rambam emphasizes learning Physics and Metaphysics [i.e. each on a set of books written by Aristotle]. The Ramchal puts down all secular disciplines and is opposed to the Rambam in this issue. why are these issues not dealt with? and what about borrowing from Aristotle and Plato that Musar does but today this is ignored?
In spite of these complaints I agree that Musar is very important for religious and non religious Jews. We all need Ethics and to understand the basic point of view of the Torah concerning world view issues. Halacha gives us "Particulara." Musar gives us "Generala."