An advantage of teaching a class frontally, as most classes have been taught since time immemorial, is that you as the teacher has total control over what information is taught. Notice that I said taught not learned. We can never be 100% sure whether or not our students are learning, and we are often not even sure what we mean by learning. However, we can be 100% sure that we have taught something, and that often provides comfort to us as teachers. It is comforting to know that if our students grow up not knowing the function of a mitochondria or the purpose of a semicolon or the correct dates for all Jewish holidays, it is definitely not our fault, since we are sure that we taught them that information.
The problem with this approach is that it makes real learning into almost a non-sequitur. Students tend to get so focused on the details that they miss the overall idea behind the learning. How much of studying is about memorizing lists and flashcards - in other words, bits of information that can be easily digested in quick shots without taking the time to step back and appreciate the forest that all of these trees make up?
But are we ready and willing to make the sacrifice in the other direction? Are we OK if students miss some details here and there as long as they are focused on the big picture? I think that this is one of the key questions that one needs to consider when constructing a PBL unit. If you are going to let students loose on information for several weeks, you could have them fill out homework questions and take quizzes every day or so, thus ensuring that they have at least written down every oh-so-important detail at least once. Or, you can have them decide on their own (with some help and guidance) which details are essential to answering the guiding question of the project. Yes, they might not be able to list every single part of the cell along with a seven word explanation of its function, but they will have a very solid comprehension of the cell as a whole.
And, let's face it, that is really all that they need. I am not trying to minimize any particular discipline, and so I will take my own as an example. I am currently involved in a PBL unit on korbanot (sacrifices). When I taught the unit frontally, students needed to know every part of the altar in the Holy Temple and what function it served, among myriads of other details. Now that I am turning the learning over to them and have decided to not give them small summative assessments along the way, I am risking that some of them may not know every one of those parts. However, they are far more likely to have a deeper understanding of how the altar as a whole functioned in offering of the daily sacrifices (for the uninitiated, it is not so simple).
To take it a step further, if I am truly concerned with my students leaving this unit with some knowledge of the sacrifices and the role that they played in the religious life of Ancient Israel, do I want them clinging to fading memories of a few scattered details, or do I want them feeling that they have a basic understanding of this aspect of worship - and that they can go back and fill in the details later if they desire to pursue it further? I am gambling on the latter option.