> In those situations the Montessori classroom's weakness is that there is only one guide for all subjects as opposed to a traditional school's subject-specific teachers.

This is very much dependent on the school (and probably age of the students). At the one my kids go to, as soon as they’re in 1st grade, they have multiple teachers: science, music (which doubles as theater - the entire school does an annual play), art, and P.E., in addition to their main teacher.