Cerana and Millifera do not really cross-breed.
Since they have overlapped in Russian Far East, they had lots of time and chances to cross-breed.
Well, no viable off-spring was found so far.
I have supporting documentation but will not bother looking for it now.
All in all, the "Cerana solution" will be like the "fox solution" when foxes were brought to control the previously imported rabbits to Australia.
How it all ended we all know.
For sure we should know these facts before attempting more of the same again - it does not work and is harmful.