Disorganized and overworked professor causing disappoinment and headache About two years ago I finished my master thesis. At our university we can only choose the topic to work on and mostly it is part of a Phd program. 

My topic was a mess because the faulty study design did not allow me to draw any firm conclusions, i.e. I could not answer the research questions directly.

However I discovered other interesting facts which are directly related to these questions.

Two things happened: 
My advisor did not want me to include these other interesting facts, presumably because one finding was a critic of a method he often used, and probably because I openly criticized the study design.
My advisor gave me the lowest grade which still passes for the thesis even though I incorporated all of the advice he gave me -- with the exception that I did not want to exclude the critique of the method he suggested.
He told me that I focused too much on the details and he had a hard time seeing the big picture. However I could not answer the research questions directly (i.e. the big picture) because of the faulty design. (Which I told him repetitively.)

A Phd colleague of mine read my thesis and thought it was great. Other students also had problems with this advisor and one phd student even quit partly due to this advisor. 

I personally do not dislike this professor but he is grossly overworked to the point where he is not even able to thoroughly plan a scientific study, causing much disappointment and headache. 

Should I inform the department administration on this matter even If I am not at this university anymore?
I don't think there's anything wrong with voicing your concerns, but if you're going to you need to find a way to do it constructively while sticking to the facts.  "Much disappointment and headache" might be entirely correct, but this is more of an emotional comment.  

Also: are there other students you can get to join you in voicing this concern?  Maybe you could all approach the problem together.