Even in a waterproof bag I would still wrap valuables in plastic bags like newspaper bags. If not that use dry bags or waterproof pouches as well.

Arkel makes some great panniers but I cannot speak to their handlebar bags as I have no interest in one currently (my panniers top zipper compartments keep the few items I need close by, close by) I do know Ortlieb also makes high quality stuff and their stuff is certainly waterproof if they say it is. There are a lot of folks here and elsewhere in the touring community who have Ortlieb stuff and it has seen them through plenty of wet weather and touring.

Nothing is perfect however and things can happen. That is why I wrote that first bit about using plastic bags anyway or take less stuff that isn't repairable or useable if wet.