Sometimes, if I press and hold the FF or rewind button, it will perform the appropriate action until I let up. Other times if I press and hold, it moves 10 seconds and stops then I have to do it again.
There are many times that WANT to move through a large portion of the video. I've mainly experienced this in YouTube videos -- I haven't needed(wanted to use these features in other videos.
Anyone have any ideas why it does it sometimes but not others?
It depends on the channel/app and what the developer decided to do. The Roku standard way is to press the button once and it starts FF/Rewinding slowly, press it a second time and it speeds up, press it a third time to reach the maximum speed. And to be clear, Rewind/FF looks like << or >>. It sounds to me like you may be referring to the left/right arrow buttons that can also be used to move through a video, but the developer can still decide what they do.