I just help friends to switch from a mailing list to XenForo, and most things looks fine, but I got some feedback on responsiveness (correct noun?).
I use a OnePlus One. Opera shows the new forum in a mobile design, scaled to the screen. Chrome displays the new forum in a mobile design as well, but the scaling is too high, so parts of the screen are hidden on the right side of my phone, so I have to scroll or change the scaling by pulling it together.
A user who uses an iPhone reported me that on his device the design is the same as on a PC.
From what I've seen in the past, I think it is not a XenForo issue, but some advice on how to fix this would be nice. (I've set Opera on my OnePlus One to "mobile", but didn't find a similar switch in the Chrome settings, but assume that Chrome is set to "mobile" as well as it shows a "mobile" design.)