Does using MongoDB for field storage limit functionality?
its a cool question.

Im on my 5 week adventure of playing around with Drupal 7 and mongo,

However as Cahpabu stated and quoted.


  NO RELATIONSHIPS WHATSOEVER. EFQ Views do not support relationships
  because Mongo doesn’t support JOINs." - Under the Converting the Views
  section on this post. (Just to clarify, I'm not being argumentative,
  I'm genuinely curious) – Chapabu Sep 6 at 15:57


I think you can do a lot using mongo with Drupal, moving a lot of other stuff into mongo, like sessions, etc. simple "fields" however :  

Since Views is extremly limited with Mongo db. I found out that its better(and much more timeconsuming) to rewrite some custom modules fitting into the schema of mongo. 

So its for sure, a great adventure if your starting up a new project/site. But the adventure will be filled with great challenges down mystery lane.

Even the Whitehouse had to go back to Mysql from mongo. 
here is a quote I found on it. 


  "The current release depends on MongoDB. When we first created the
  application, we wanted to make sure we had a highly scalable
  application and database to meet our anticipated performance needs
  under high loads. We have been running MongoDB in production for over
  a year, but we have decided that the performance benefits it provides
  are outweighed by the complexity of trying to extend Drupal features
  backed by MongoDB."


However other people had great succes with mongodb and Drupal 8
Mongodb and Drupal 8


  As they state in their slide, To achieve full mongodb functionality is
  of course impossible.


But you will have to give up several modules using SQL, (which is a lot) For now, I still have the feeling that Drupal cant dance a full tango with Mongo. Even using the modules. its still a rough way.