You are right, they wouldn't be that bad, they would be ABSOLUTELY F*****G AWFULL! Honestly, taking two armies, who already have their own distinct background, more than enough troop types to field valid and vaired armies, who have completely different playstyles and mashing them together is a ridiculous idea. The only connection they share is that they happen to be neighbours. So on that basis, why not do Lizardmen and Dark Elves!? Dwarves and Skaven!? The only possible redeeming feature for this idea is that you would end up with armies consisting of the best cavalry in the game backed up by the best archers in the game and thus have game breaking super armies. But even then, Bret and WE players would still be mighty unhappy (as would everyone else, I hasten to add).
Which is why I'm hoping I've misunderstood this and that someone can shed more light on it rendering my current nerd rage premature.