I actually think (and have personally observed) 40 and 80MHz 5GHZ channelization works in a lot of cases, especially with ChannelFly active on 5GHz. In fact it sometimes annoys me that moderate density enterprise deployments prematurely select 20MHz thanks to some sort of "best practice" guide or regurgitated myths.
80MHz channelization is especially helpful for 1 and 2 stream mobile clients achieve 100+mbit wireless rates, and tend not to be as bad as it's made out to be in terms of co-channel interference since most of the times, clients have a finite amount of network traffic they want to do (e.g. upload 10MB picture, download 100MB app update), and if you let them do it faster with fat channels, they will get off the channel faster too.
Well I think it's hard / misleading to sell a 802.11ac AP and have the default configuration be half (or less) of the advertised bandwidth/speed numbers!
80MHz cuts the number of nonoverlapping 5GHz channels down to 6 (assuming you are in the US and are allowed to use all the DFS channels), which is still a lot, but is probably not good use of the spectrum for extremely high density deployments.
FWIW unless you're running like a stadium or concert where you legitimately need that many AP's in the same area to service clients packed closely together, then 20 or 40MHz channels are probably more appropriate.
But I'm right now in an office environment where the IT dept chose 20MHz AC channels because they have a ton of (Cisco) AP's in the hallways and wanted to avoid overlapping channels, except the density of AP's is actually for coverage area, not for high # of clients. Sniffing the air waves almost every 5GHz channel is unused and I put a Ruckus R700 on a 80MHz channel that's "overlapping" 4 of their AP's, and I can consistently achieve 450mbit up and down while against the 20MHz Cisco network I struggle to pull 90mbit.
It all depends on how many clients you expect to serve, and what's more important —increasing the # of clients you can service, or delivering extreme speeds to the clients you have.
Channelfly in theory should help out a lot if neighboring AP's are sharing or overlapping 80MHz channels and one suddenly becomes busy. Channelfly will recognize the sudden loss of capacity and hop to a more available channel.