Thanks for info.
I am not sure about whether this relates to sync (between TD and genlocked video I/O card) as I haven’t managed to do a proper test, but in general I spent quite a lot of time working with Blackmagic cards and I have noticed that even when using very simple scene, I can very occasionally see a frame drop on video output. This might and might not be related to this topic, but I though I might share it here. It isn’t something that could be easily spotted as it usually happens once in a whole day. Obvious conclusion would be that there is a performance issue - some CPU / GPU peak, many operators cooking at the time. However I sometimes feel like this isn’t the case as the scene itself is very lightweight, hardware is powerful enough and not a single frame drop happens in hours. And then it suddenly drops one frame out of nowhere. Seeing this happen many times over last years, I started to wonder if this could be related to sync between TD and video I/O card. But as I said, so far I haven’t been able to test this properly and it could be related to something totally different. My question is, please do you think this could be happening because of not having TD synced to video output?
Another part of my wonders goes to latency as there is a constant struggle to minimize this as much as possible. 
I am not sure if sync between TD and video I/O could help in this area, but based on what you said I feel like it could make a difference, right?
I guess there is probably some reason for which Unreal decided to go this way by adding option to sync the engine to genlocked video I/O card.