I think there is a difference between chance at a detailed level in the game – does this unit do well or badly in this combat – and chance at a high level – do all my units suffer a disadvantage, does half my army run away. Most of the games I play seem to have relatively high amounts of chance of the first sort, but low levels of the second. You can make a plan, and find chance gets in the way, but no one unlucky roll makes impossible your hope of a victory.
I can understand that the second can often be justified historically. There were times when armies were much worse than expected, or when the rout of a single unit caused half the force to collapse. Maybe if it is part of the scenario conditions, it can work. However, it seems to me that any such major chance event is likely to be to the detriment of the game as a contest (however friendly) between the players.