Nothing acidic about Monsieur Balmain. It's very lemony, true; but it lacks the bold underpinnings of the YSL PH scents. On me, just my recollection, it stays mainly a lemon/citrus scent, and towards the end of about 5 hrs or so, the woody basenotes come about, and ultimately, my skin pH works through and makes the scent seem like a very light verbena/musk scent, but by this point, it's SO close to the skin. I can't speak for many here, but I have a feeling that the "sharp" they are referring to is in the sense of how much citrus is present. I will assure you I have not sensed anything putrid or nasty about Monsieur Balmain. That's not to say that the initial blast doesn't smell like a lemon, freshly sliced, placed up to one's face.
I feel that the use of the word "sharp" may refer to the boldness of the lemon.