Much of this 'aggressive crosses' stuff has its origins in the *opinion* of some heavy hitters. There are one or two much quoted text books that are actually near to racism, albeit projected onto the bees, and just about as sensible.
Over tens of thousands of carnica/black crosses I can say with total confidence they turn out...for the most part...as intermediate in every way.
Over rather fewer, but still four figures, of buckfast/black crosses. Similar result.
Have done many deliberate Buckfast/carnica crosses in the last four years and find no issues with them whatsoever. Nice easy to manage bees.
However....it is easy to dismiss a colony as aggressive when in fact it may be as few as one sub family that is to blame. If 5 to 15% of the adult workers are nasty then you can think the entire colony nasty. Requeen it from its own progeny and there is a high chance that the new queen will come from a sub family that is not aggressive....so yes...a colony can change.
What is undoubtedly the case in our area is that everything, in time, reverts to black bee types. These can throw *some* nasty colonies in their crosses.
The thing that amuses me is that the black bee lobby say that when you cross a calm bee (carnica or buckfast in this conversation but could be others) with a bee of uncertain temperament, and the cross turns out unpleasant that they blame the calm side of the cross...............
The idea of restoring the UK (or Ireland for that matter) to an Amm heaven is a multi generation project and a pipedream, with uncertain results........to restore a bee which, until modern revisionism sanctified it, was considered 'difficult' by its keepers.
As a child with my father in the early 1960's we used to visit some (even by then) very elderly beekeepers who had been around pre IOW disease and had old native blacks in the glens of Aberdeenshire. They much preferred the 'new' bees to the old ones.