Your project, say — would be done now would only MongoDB. The main problem is search. At the same time to do a search on one database and another, and how to find the intersection of data? That is, we need to find the users profiles (SQL with constraints) with red noses and green ears (Mongo). In the end, we need to do or a request to the RDBMS, and then the obtained id to search for Mongo with settings or Vice versa were found in Mongo parameters to look in MySQL (or other database).
If the data bit, then no problem, but when the number of similar data increases, and increases the number of records that need to carry from one base to another.
Now the search will move to more ideologically kosher Sphinx, but the project will be to rewrite the full MongoDB.