The major--and I mean major-- problem with Musar is lack of philosophical sophistication--[i.e the movement started by R. Israel Salanter to influence everybody to learn the texts of Jewish Ethics written during the Middle Ages]. [That is the movement was founded on Ethics and almost ignored the thought behind the Ethics. That is: it needs to have the mediaeval philosophy of the Rambam and Saadia Gaon to back it up.]
The two essential works of Musar are the Obligations of the Heart (by Ibn Pakuda) and the Light of Israel (by Isaac Blazzer, a disciple of Israel Salanter).
This is not to imply that the texts themselves lack philosophical sophistication, but that the whole movement morphed. Valid questions are ignored and by ignoring these questions we think we have answered them. Or we accept foreign ideologies and try to claim that they are what Musar says. And even worse, the failure in the one thing it is supposed to be doing--character development.
The philosophy that backs it up is in the Guide for the Perplexed, Joseph Albo and Ibn Gavirol that people do not learn.
That is: the Musar of the Rishonim is highly linked with their philosophy. If you do not know this, then you can't even begin to understand their Musar.
What are you going to do? Just teach Musar and assume people will fill in the blanks with the basic philosophy behind it? Or you think people will not worry about justification for the ideas? Teaching Ethics from the Middle Ages is a great idea--but only if it is coupled with the philosophy of the Middle Ages. The Guide, Saadia Gaon, etc.
Maybe I am too ambitious here as to what people can reasonable be expected to learn.
So to make things simple get one book of authentic Musar [Obligations of the Heart perhaps]. And one book of world view like the Guide of the Rambam.
Simcha Zissel from Kelm is one well known disciple of Reb Israel Salanter. In his yeshiva in Kelm people learned Musar most of the day.
There is another possible flaw that the movement was not coupled to practice. What better way to learn good character except by teamwork and outdoor skills. Self reliance, keeping your word, loyalty, working together a a team,- how better to learn these things except by camping and working on survival skills in a group? The general result of Musar without this outdoor component is that people that learning Musar seldom can be relied on to keep their word or help one in time of trouble.
Musar is just as important as Reb Israel Salanter said but it has to be coupled with outdoor skills and team work and also the philosophy of Torah
Ethics of Torah= Musar. Many aspects are contained in the Mediaeval Musar books.
The Sexual Ethics of Torah is not the same as Catholics understand it. Sex with one's girl friend is allowed. We find this with King David and Solomon and Calev Ben Yefuna in Chronicles where the list of his wives and girl friends is given.
See the Book of Chronicles, Volume One, chapter two [2:46] and four concerning Kalev ben Yefuna (כלב בן יפונה). He is the only person in the entire Bible that it says the unique phrase, וימלא אחרי השם "He walked completely after God." He had two girl friends.]