Honestly, 80MHz 5GHz channels is often an underutilized feature because of how scared we are to use it. If you are in a relatively RF-clean environment where you don't see more than, say, 10-20% utilization in your RF Spectrum, 80MHz channels tend to work. This is especially true in home setups or small office sort of venues.
80MHz theoretically doubles your throughput compared to 40MHz but even in practice, it is one of the best ways of getting improved throughput on 1-stream and 2-stream devices. Right now in my home office, even on a sort of busy channel (36 with 4 neighboring APs above 80dBm), 80MHz channels on a MR56 with a iPhone 11 Pro gets about 400-450mbit while 40MHz channels gets me 250-300mbit. If you have clients that mostly burst downloading large files, this can make a huge difference in customer satisfaction.
It's also worth mentioning that even if you select 80MHz channels, both APs and clients are welcome to select 40MHz or 20MHz rates. There are rate control algorithms on both end that won't even choose 80MHz MCS rates if there's not enough SNR for it to be successful.
With that said, where it starts going wrong and these things must be kept in mind:
- Without DFS channels, you basically have 2 nonoverlapping channels in the US (36-48 and 149-157)
- Having multiple APs on the same channel is not necessarily evil for low density. It's only a problem if multiple neighboring APs have high traffic, but if you have a lot of APs for the purpose of increasing high-RSSI coverage for maximum throughput, I've had great success running even two APs in neighboring rooms on the same channel when there's just one critical client that moves between those two rooms.
- With DFS, you have a few more nonoverlapping DFS channels: 56-64 and 100-112. But you may not even have 136 in your area depending on whether or not your AP supports those channels neighboring weather doppler radar.
- Furthermore, you greatly increase the chances of a DFS radar event when your channels are wider.
- If you start having co-channel interference issues where you have high or maybe uneven usage of your 80MHz channels, at that point you have no choice other than to step back down to 40 or 20MHz channels
I cannot tell you how often I site survey an office environment and see dozens of APs for an office environment all set to 20MHz channels and less than 10% BSS load on any of the 20MHz channels. There goes your 400-600mbit wifi throughput, you just limited everyone to 100mbit or less.
Of course there's situations where that's the right thing to do but the sayings around premature optimization applies to wi-fi too. It's not always the right knee-jerk reaction to read a "Ultra High Density" best practices guide and assume that your own network has got to be ultra-high-density too.
Luckily, Meraki gives you lots of great tools to take a look at your RF spectrum and also take a historical look at how many active clients and what kind of utilization/throughput is typical on your network.
EDIT: One issue I'd like to call out: AutoRF channel changes basically seem disabled when your channel is 80MHz. This is documented on the AutoRF documentation page with a description saying that clients disconnect/reconnect on 80MHz channel changes but this does not appear to be the case when I manually force an 80MHz channel change. Hopefully Meraki can consider revising that behavior for 80MHz.
