The current doc page (http://www.mongodb.org/display/DOCS/Server-side+Code+Execution) mentions that you cannot use write operations with $eval, but it does not mention that you should not use any features that requires the Javascript engines - for example, $eval, $where, map reduce.

Not sure if this is also the case for our V8 engine, but to use the Spider Monkey engine, the global Spider Monkey lock must be acquired. This means that the server will get into a dead lock if you try to call any features that uses the javascript engine.