When your boss wants to use Mongo and Node because its trendy and popular.

somebody please rape his boss in the mouth. thanks

No point using a framework if it doesn't match your requirements. If it's integrating with an existing system an impact analysis would probably indicate that doing so would be a reasonably risky task with potentially disastrous outcomes. If the only justification for it is 'because it's new and trendy' then kick it into the long grass. Justify your concerns in email (don't do it face to face) you want an email trail of your objection !

Alternatively, if you are being forced to use it. Make the shit out of some lemonade with those lemons.

I'd kill to have a paid opportunity to gain exp of node + mongo.