Credit Karma uses Trans Union. Recently they started offering free credit reports. They must be getting a ton of income from the credit card and mortgage referrals to afford that, even with the deal they probably struck with Trans Union. Credit Sesame uses Experian. It's useful to check two scores and subscribe to both alerts, so any fraudulent activity I miss will get picked up by one of the two credit bureaus. Credit Sesame is trying to move everyone to a subscription model. They make it difficult to wade through the subscription advertising to get to what you need.
I do not link my accounts. One more way to get hacked, don't want that. I'm also concerned about the potential mingling of income and spending information with credit information. These folks could make a mint by creating an all inclusive consumer profile and selling it. There are enough Big Data collectors out there already, why make it easy for them?