0 "Dear Concern" and "Dear Concerned" are both far from idiomatic. I believe the phrase you are looking for is " To whom in may concern " which is a common email salutation when sending an email (but especially when writing a letter) to someone of whom you do know know the name.
The alternative you have suggested, "concerns raised about..." can be rephrased to "concerns that are raised about..." So the main noun isn't immediately followed by the "about" clause, which could be misleading but it doesn't change the intended meaning. It may not be stylistically correct in some people's eyes. It's just a matter of approach sometimes and how you decide to interpret words to ...