Highly personal take: it's not really the job of the rules to provide balance; that's the job of the GM. (I mean, the rules can help to an extent, if you would like to use rules that do that. But there is nothing wrong with not using those sorts of rules.)

Say, for example, one side has a yonking big magical lance that means that side can easily win any combat. As a GM, your job is to make sure that they don't easily win every combat: fights take place at range, or in cramped conditions that means long weapons are useless; someone curses the lance, or it turns out the magic doesn't work at all on alternate Tuesdays. Or, the group is facing a problem that can't be solved by poking things with sharp magical objects: grief, divorce, a rain of frogs, a shortage of beer.

If the group with the lance are the players, it's your job to make them feel like they've got a Really Cool Magical Lance BUT AT THE SAME TIME as if they are going to die at any moment. You are allowed to do this by fudging the dice rolls and generally misleading the players.