I use bitcointalk on mobile and honestly it works perfectly fine for me, yeah you have to zoom in, zoom out a few times but all the buttons work perfectly fine. I use an Iphone 6.
A responsive layout should be the fundamentals of any websites in current times. Scrolling left and right, pinching your mobile screen are not the norms in this era.
But if security of this board takes priority, i'll say focus on security.