OK, I will concede to the point of your friend who breeds macaws. I have heard the same thing from one of my friends that breeds macaws and I have heard from several other macaw breeders why they shy away from scarlets. After hearing all the negative things associated with scarlets, I too shyed away from them for years.
There are also well experienced macaw keepers that recommend strongly that scarlets are only for experienced macaw keepers that know up front what they are getting into concerning scarlets and their nature.
While agreeing with all of that, I still stand by what I said about scarlets in my earlier two posts.
Scarlets 'are more' on the spastic/unpredictable side with using their beak and the way they tend to vocalize...and I will certainly agree 100% with saying that scarlets are NOT for everyone, including experienced macaw keepers.
To me, you're either a scarlet person or you're not. One person will like them for the same antics that makes another person back off.
Bottom line 'to me', they get a bad rap because they are highly misunderstood.
Perhaps I say that because 'I am' definitely a scarlet macaw lover to the core and find them absolutely fascinating and enchanting.
The biggest problem I see is that folks have heard the 'nippy' and other labels so many times that they go into reacting with a scarlet with that in mind.
I don't see the nature of scarlet macaws as being a problem at all. I see people's 'reactions' to their nature as being the problem.
Scarlets are highly intelligent and calculating. As my friend always says, they are always one step ahead of you. If they get a reaction out of you based on what they are doing, they will play you like a fiddle.
Just my personal opinion and experience...my answer as far as making a 'temperament difference' is that it doesn't make enough of a difference to count one way or the other.
As with any other macaw, the way it is raised and socialized will tend to guide it's temperament and reactions to people and it's environment. And above all of that, you still have the 'individual unique personality' of that particular bird.
I won't get into all the hybrids, but in the case of ruby's, breeders prefer/try to set them up with the greenwing as the male of the pair with the scarlet as the female of the pair.
The main reasoning behind that is that the male 'should be' the dominant gene. (I can't validate that claim tho) With that in mind, the breeders 'are' going for the gentle nature of the greenwing along with the color of the scarlet added in.
It is also an 'appearance' thing. Once again, 'with the thought of the male as the dominant gene', the thought is that you want the male of the pair to be a greenwing so you will have a 'greenwing with enhanced coloring'. The scarlet being the male of the pair is shyed away from because of the chances of the babies looking like scarlets with bad coloring.
