I find the USG one of the easiest devices to setup + the documentation is good.
I'm not quite sure where the reviewers have a problem
But then I have setup a few....
We bought USG50 and it is working very well, solid, not a lot of drops, and combines bandwith down (up, not so much, but it may be due to policies).
The only problem with USG50 is the VPN, it is very painful to setup VPN on this device, unless you pay for VPN client, even then it is still difficult.
Also firewall is extremely advenced with no way to make it "easy" but I guess it is a good thing for those who want complete control over their rules and setup.