The Streambar devices do use a different processor than the latest players. I can't say if that's a good or bad thing, as I've never tried using any version of Streambar. But the OS is basically the same, and should only differ for hardware differences. But the hardware does differ quite a bit compared to the latest Ultra or Express 4K+.
I do have to say that Roku hasn't made any noticeable updates to Roku Media Player (the video player app) in more than three years. It has many "quirks" (to be polite about it) with video playback, but my experience with MKV files are usually good. But video and audio codec differences in your files might explain why they play well on one device but not another. Remember that the MKV is merely a container, and the objects within that container can vary significantly. Something as simple as the compression level of an H.264 video might cause a problem.
Dan
Roku Community Streaming Expert
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