First, I'm far from being an expert ;D
I think this is pretty much dependent on the results you're looking for.
Anything that gets thrown in primary fermentation will be 'blended' into the mead. It'll show as one of the complex aromas and aftertastes in your finished mead.
So this is a good time to add the items which have the most strong tastes, or the ones that you want to appear more blended than singular.
Fruits with subtle or light tastes such as clementines or bananas will almost completely disappear, taste-wise, when used in the primary.
The secondary is the time to add what you want to stand out individually in your mead. It might be strong tastes (cinnamon, anise, peppers, chilis) or subtle ones (clementines, bananas, watermelon, ...)
Another reason to use some additives in the secondary rather than in the primary would be that they have a negative impact on yeast viability and/or fermentation. PH comes to mind here.