As a smaller channel that has received countless "invitations" (read: spam) from MCNs, I can firmly say that this topic pretty much outlines why they're not a great idea for most people.
From a personal perspective, early on in my YouTube journey I used to ask people that asked me to join for details. Basically, what they were asking me to agree to in writing. A lot of times, I would get no response. Sometimes, I would get a very generic document that left me feeling leery. Never once did I get something that looked better than going it on my own through Google AdSense.
If you're a more successful channel, chances are you're going to get better results than I did and better offers. But, as a smaller YouTuber, I don't see any reason to join a MCN that requires you handing over your AdSense credentials.