"Well now, we had a deal. You get to sing songs sweeter than ice tea on a summers day and I...I get you."
A weakness that is true to the character, a deal that seems amazing (you get Mystifying Song) that turns out to be not even close the price of purchase. Mystifying song, in a vacuum is an extremely powerful signature card, especially paired with arcane initiate but by god, is Baron a DEVASTATING weakness.
I would be so bold as too say that he has to be one of if not the WORST personal weaknesses amongst investigators to the point that he has played a large role in Marie's lack of play-ability. The jankery you need to include and draw in your deck to handle this weakness is absurd. If your lucky (scenario pending) you can handle this in the first 3-4 turns, if not...You're screwed.