There was a recent post with a similar question (though, about damage rather than mana).
Aside from some heroes (e.g., Proteus) with the ability to suppress mana generation or (e.g., Li Xiu) diminish it and the fact some troops actively improve the rate at which mana is generated while others don’t, I have never noticed a disparity in raids. (I’m not saying it doesn’t happen, I just haven’t observed it.)