I've found another issue - if you set the "Apply Method" to Deferred then the ssao produces no visible effect if the ambient colour is set to black - and is pretty much invisible for any dark colour.
I've noticed this because my game's environment is very dark and I have ambient pretty close to black.
Again, this might be deliberate but it seems wrong to me - with it this way any pretty much any light is enough to remove all darkening caused by the ssao.
The bigger issue for me is that this means one set of settings for the SSAO won't work - for example - when moving from a light outside into a dark building.
- Unreal Engine