Is it just the one client with this problem?
There was a recurring problem with Ruckus Unleashed releases, where some clients get slow download rates. I guess the latest release of 200.7 has broken this again? (gah!)
I just whacked your Unleashed version on an R600 and set my country to US, enabled optimize for performance, and setup a 5ghz-only SSID. And I connected using a Surface Pro X, which has experienced the problem in the past (nothing else I own had speed problems).
With optimize for performance, I get very poor download speeds from fast.com (<200mbps). If I choose optimize for compatibility, then my download speeds are ~460mbps (it has chosen channels 149-161).
Sooo.... maybe try using the optimize for compatibility setting.
(My country doesn't have this setting, and besides, I usually set non-DFS channels when I setup an R600 for anyone since Fire TV boxes have historically refused to work on DFS channels).
Regarding the 2.4Ghz band, it's not so stupid as it seems.... I believe ruckus channelfly just tries out all the channels and picks the 40Mhz slice with the highest throughput. If you put in another overlapping 2.4Ghz AP nearby then the interference will make channelfly jump around find a better 40Mhz bit of spectrum.