We were on 6.5.3.3 instant and ran into scalability issues with Airgroup. The instant controller couldn't handle all the airgroup traffic, specifically for hundreds of Chromecasts so we had to purchase controllers after working with support onsite.
The question is has the scalability changed since 6.5.3.3? We are planning on moving off the controllers to 8.6.0.6 because the controller licensing upgrades and renewals are just too much.