Previously, $comment was permitted to be any type. Per
SERVER-27438, 3.4 changed that so that $comment is converted to a string internally. This breaks application that were depending on $comment being a document (in our case, we store things like caller information and tracing spans in $comment). We extract this information from the system.profile log and inprog.
Separately, it seems like the comment field in the find and aggregate commands should similarly accept a document.