I used one of the 19.07 images they provide, not sure which version exactly.
I did hit the issue with flushing, but flushing it in a separate command before loading the rules worked for me. I have not tried anything more advanced as QoS, just a minimal configuration to show that the stateless NAT works.
I recently upgraded some routers to 21.02-rc2 and it seems to work nicely for me. I’d suggest to give it a shot if nftables on 19.07 doesn’t work as expected for you. Some major Linux distributions already switched to nftables in the mean time, so I’d expect it to be well tested at this point.