Well from my personal experience.
Don't even try to cram 7 BE into 7 port USB 2.0 hub. It will never work (worst case scenario - it will burn hub/usb port on motherboard or USB card). It happened to me once when I plugged 7 into 7 [active] port hub and it fragged NEC USB card. Tested few and 5 is max for stable work with USB 2.0 hub. With 7 I've burned pci usb controller, but before it fried 2 out of 7 returned always 100% HW errors.
Of course cooling is a factor. Hot chips usually bang too many HW errors. Nothing fancy slow fan will do just moving hot air.
Best way with plethora BE is with really
big USB 3.0 connected active hub. It will support much more hardware and power shouldn't be an issue. I went with Manhattan MondoHub [161718]. Pricey, but hey - 28 ports 4x3.0 and 24xUSB 2.0. Practically usable 2x USB 3.0 and 18x 2.0 (obscured side ports, too bad that BE boards are so wide). I will test at later date if it will detect BE properly when plugged into obscured port with extender.
My small, temporary station, not completed yet. For now connected to main PC [W7 64/BFG 3.1.3].
http://s22.postimg.org/if8au5h9d/DSCN8799.jpgThere is for certain one picture somewhere showing 30+ BE working without problem. My guess in your case - insufficient power. Ditch USB 2.0 hubs and try couple USB 3.0 - the only way to supply stable power to so many BE sticks.