With a Direct Publisher channel you are allowed to provide a single ad tag in your Roku Developer Account, the ad timing is preset by Roku, and you need to operate your own ad server (at least a few hundred dollars per month, typically more) to obtain ads from multiple sources in order to get a decent overall fill rate. If you are not running your own ad server and you are using a single ad tag for a Direct Publisher channel, then you are leaving a lot of money on the table, as the fill rates from any single ad provider are usually somewhat low.
With an SDK channel you can run ads from multiple ad providers simultaneously, allowing you to have a better chance to fill all of your available ad inventory. SDK allows to you have greater control over ad timing - when the ads start to play, min and max times between ads, and so on.
Ads often pay out less than a penny per ad play, for example 0.6c to 1.2c per ad play ($6 to $12 CPM), possibly more if you operate your own ad server. If you can get an ad account with Roku, it's been my experience that they will pay towards the high end of that range, or slightly higher. Barons pays more towards the middle of that range, but they're a bit easier to get an account with and (at least currently, for me) they fill much better than Roku does. The most successful devs I'm familiar with who are monetizing with ads will run dozens of Roku channels at the same time, dropping the less profitable channels and adding new channels every few months. With SDK channels, or with Direct Publisher channels using an external ad server, you will typically have multiple ad sources in a channel, configured in a "waterfall" so that if the first ad source does not fill an available ad, the remaining ad sources in the list can be automatically queried until an ad is filled, or until you run out of ad sources in your list.
I believe that the reason why it's difficult to get started with Barons & Roku is that it's expensive for them to set up ads for each new Roku channel, to monitor the performance of the channel, to generate the daily or monthly revenue reports, and to do the bookkeeping required to issue payments. They want to make sure that the channels that they work with will generate enough revenue to offset all of their one-time & recurring costs.
Hope that helped!