Xfinity Streaming TV and Internet Data Usage ...

Just as you describe, Comcast has been very inconsistent about what does and does not count against the Data Plan (see https://www.xfinity.com/support/articles/data-usage-plan). My understanding is that content viewed on a Roku using the Xfinity Stream app does not count against the plan. But if you were to load any other streaming app on the Roku and begin streaming that content, it (probably) would count against your plan.

https://www.xfinity.com/support/articles/xfinity-stream-beta-app-faqs says:

Does streaming using the Xfinity Stream app on a Partner Device count against my Internet data usage?

No. The goal of using the app on your Partner Device is to give you more options outside of our rented TV Boxes to access your full TV service within your home. The service delivered through the Xfinity Stream app is not an Internet service, so our data usage plans don't apply.

However, video services that are delivered over the Internet, like TV Everywhere content accessed through the Xfinity Stream app and web portal or content from NBC.com or the NBC and NBC Sports apps, are treated exactly the same way as any other Internet-delivered services, like Netflix, Hulu, or Amazon, and are subject to any data usage plans that might apply.

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