Ludington, Michigan is an example of where the route is bidirectional but would most certainly benefit by being “improved” (to have more specificity, to avoid “forward/backward conflicts”) into two unidirectional routes.
But it isn’t that USBRs AREN’T directional, of course they are. Some are entered as a (relatively simplified) bidirectional route, some are entered as two unidirectional routes (tied together with a super-relation). It is how we denote (tag, and document as how we tag) these that is different than road routes. No USBR (to my knowledge) that is either unidirectional (and hence shouldn’t have any role relations on its members) OR bidirectional (and DOES or CAN have role relations), has role tags BESIDES forward and backward.
Yes, road routes have west/east and north/south tagging conventions (in some cases) as you call out the documented way to do this. USBRs do not. And while we do not explicitly say so in our wiki, maybe we should, to clarify this distinction between national bicycle routes and road routes. It seems that doing so could have cleared up this (actual or potential) confusion!