Speaking from my experience, despite the things you mention missing from the 5GHz radio, side by side comparison between my R600/R700 and the ZF7982 in two sites consistently show that the AC stuff has superior 5GHz throughput whether you're close or far away from the AP. It would definitely be nice to have ATF, background scanning, and meshing but even without that, the performance is significantly improved in my experience, and the reliability is about the same.
I've not seen clients mysteriously drop off. I had some broadcast/multicast reliability issues with 9.8.0/9.8.1 standalone which seem fixed per the 9.8.2 release notes, but with a ZD it's been consistently solid since the R700's release.
Can you elaborate on your 5GHz issues (such as how BG scanning / channel fly don't work, and specifically what kind of client drop issues you are noticing with which devices?), because as you mentioned, it's kind of unfortunate if you have to disable 5GHz since all of the advantages of R700 are with its 5GHz radio.