If I was into Burton gear, I feel like I'd want to pair a channel board with est bindings, period. Seems like you're not utlizing the benefits of their setup if you don't run them together.
The Channel is more about stance options and ease of use.
EST is more underfoot flex, same with Re:Flex just a little less of it.
None the less, some people have a different rail board, a pow board, a rock board etc which may not all be Burton.
Or someone likes a Burton board and just really likes Union so to each their own.