Dangerous around 20,000? I think so. Swimming would be bad.
Someone with big water experience will probably have no problem in a raft or a kayak. But I wouldn't want to swim it no matter how experienced. I'd shit my pants if I swam above Skull.
The only times I've done it > 20,000 was a while back. I was comfortable on my local low volume class IV back then and I was kind of scared on Westwater at that flow. None of the other kayakers I was with were nervous so it was my low fear threshold. I asked them if they were scared because I was a bundle of nerves after a whirlpool that spun off an eddy line took me and my boat down deep for the count. They said, nope this is fun
If you are comfortable in class IV and have big water experience then no sweat. But if borderline in class IV and/or don't have any big water experience then 20,000 cfs in a kayak will be a rush for sure and if you swim in some spots you will be scared for sure.