Something I heard from a large company in Atlanta. Productivity didn't drop off a ton, but innovation did. People are effectively in silos at home and they couldn't figure out a way to recreate the collaborative effect that was seen when you have a bunch of loosely affiliated engineering teams working in close proximity with each other.
I'm not saying you can't get close to similar levels of collaboration remotely, but it's hard and it requires thought.
The biggest thing I was always pushing for was use of public channels in slack. If you silo all your questions in direct chats, you never get that "I overheard you talking about x" effect. Some companies just don't get it.