I'd say the "common support issues" will depend on the skill level of the end user - Novice or Experienced.
Novice users (i.e. people that are new to Edge products) will most certainly have issues with the Edgerouter and Edgeswitch GUI. I certainly had my share of challenges when I first started over a year ago. The interface has improved since then but it still has a long way to go before the interface can be considered intuitive.
Experienced users (i.e. people that are now familiar with the product line and want to do more) will, as has already been thoroughly discussed, have issues with documentation and the lack thereof.
All that being said, the saving grace is the fact that Ubiquiti's community is so active and helpful. Had I not received the assistance of some great people on these forums, I never would have been able to get my EdgeRouter X working to my satisfaction and would have likely returned it. I guess what I am trying to say is this - as long as the community continues to a positive, encouraging place to solve challenges, we can get by until the GUI experience and documentation can get caught up. 