Google adsense allows you to monetize your content and get your cut directly from YouTube themselves. YouTube takes 40% of all revenue made on your channel in a given month. You get 60%.
Now then, let's talk about MCNs.
An MCN is basically the middle man. They make sure you have some semblance of protection should a copyright strike appear. They help negotiate with brands to help you make more revenue, and usually they promise to make you grow. All at the cost of some of the revenue.
Now, let me be clear here. YouTube already takes 40% of your revenue each month with videos monetized. That means you're left with 60% if you are partnered directly with YouTube.
However, the moment you join an MCN, they're going to take their cut from whatever YouTube gives you. Usually, this is within the realms of 10-30% but in worst case scenarios it can be much higher. Machinima, as great as they might have once been. Have been known to abuse their partners by locking them into contracts for years.
Now, let me get to the promises!
1. We're going to help you grow!
This is the big one, that most people find appealing. MCNs usually have no interest in helping their partners grow. There are exceptions to this rule, but generally an MCN will only help you if you have proven to be successful time and time again. Though, they want to see numbers. So, don't expect them to promote you at all if you're small. They might provide a Tips and Tricks! sheet that tells you the basics of SEO and content strategy. Though past that, I wouldn't be expecting to talk to a human in your time there.
2. We'll get you brand deals and sponsorships!
This one is true. Networks will get you brand deals and sponsorships so you can earn more revenue. However, much like before they're holding these back for people who show results. This one is understandable. Brands most likely never want to promote small guys that will maybe get 1,000 views on a good day. You know?
3. We'll protect you from those mean copyright strikes!
Typically, this is also true. Though, let me just tell you that unless you are uploading movie clips or audio clips from songs. You'll likely not run into this issue for a while. The worst you'll get is content ID matched. Which is when someone starts to claim all the revenue on your video for containing copyrighted content that steps on their toes. As long as you know your content falls under the tarp known as fair use, you should not have any issues disputing copyright strikes and content ID matches.
That being said, if you feel as though it'd be worth your time getting into a network. Please feel free to do so!
Here's some things I would look at before signing any contract. Ever.
1. Revenue Split
2. Term and Termination.
3. [Networks] Services
4. The rights you give to [Network]
5. Content Guidelines
6. Ownership of channel and content.
Make sure you do NOT give them full ownership rights to your channel.
Do NOT accept anything with a revenue split lower than 70% (80% and up is what I would look for.)
Do NOT sign a contract that locks you in for more than a year.
Do NOT fall for an infinite contract loop (Really, read those termination clauses people.)
Hope this helped. There might be more things that are missing, but this should give you a general overview of what you should look and watch out for.