Why is it not recommended to use server-side stored functions in MongoDB?

Question

I am sure I have stated the list a couple of times despite the Google search result being filled only with people telling you how to do it:

It is eval
eval has natural abilities to be easily injected, it is like a non-PDO equilivant to SQL, if you don't buld a full scale escaping library around it it will mess you up. By using these functions you are effectively replacing the safer native language of MongoDB for something that is just as insecure as any old SQL out there.
It takes a global lock and can take write lock and will not release until the operation is completely done, unlike other operations which will release in certain cases.
eval only works on Primaries and never any other member of the replica set
It is basically running, unchecked, a tonne of JS in a bundled V8/spidermonkey envo that comes with MongoDB with full ability to touch any part of your database and admin commands, does that sound safe?
It is NOT MongoDB and nor is it "MongoDBs SQL", it runs within a built in JS environment, not MongoDBs C++ code itself (unlike the aggregation framework).
Due to the previous point it is EXTREMELY slow in comparison to many other options, this goes for $where usage as well.

That should be enough to get you started on this front.