I read the editorial that rockaction linked, but it doesn't have very much detail explaining exactly WHY the charges are incompetent. It complains that she didn't use some expert the writer wanted her to use, and that she made her decision too quickly, but neither of these complaints goes to the incompetence of the actual charges. This is, however, this portion:
In fact, Ms. Mosby was so hasty it appears she locked up two completely innocent officers. She charged Freddie Gray’s arresting officers with “false imprisonment” because she said the knife that Gray had on him was legal. In fact, as The Sun reported, the Police Task Force found it to be illegal after all. It was Ms. Mosby who had no probable cause to lock the arresting officers up, an injustice she could have easily avoided by taking her time.
Within the editorial, this appears to be the only factual criticism. Now if it's true, it's certainly problematic. However, there is no link to this information, and the only support I could find online is this "Legal Insurrection" website:
http://legalinsurrection.com/2015/05/confirmed-freddie-grays-knife-was-illegal/
...and the argument about the knife actually being illegal is very confusing to me; I don't understand it. I understood when the prosecutor said, "they wrote it was a switchblade and illegal, when in fact it wasn't a switchblade"- anybody can understand that. But I don't understand this. In any case, I thought that the whole point of the prosecutor's statement about the knife was to point out evidence that the police had fabricated their original report. That was my impression, and still is. So even if the actual knife is somehow illegal, I'm not sure what that has to do with fabricating the report and mentioning a switchblade, which was the basis for the false imprisonment charge. But then, I'm not a lawyer.
I'm still trying to find Dershowitz's critique. Hopefully it will be better than this one, which really doesn't make any sense to me.