I think it is highly dependent on the hub and the devices. I've never been able to get my Plug to boot through my external USB hub, although Linux has no problem accessing either one or multiple devices attached to it.
If you have a hub that the Uboot can handle (i.e., can boot reliably from USB with one device connected through it), it has been reported that reordering the devices on the hub sometimes achieves success. Others have just given up and unplug the other devices to boot, and replug them afterwards. YMMV.