up until the development of rifling, there's really very little an opponent can do against a large stack or cuirassiers. put it this way, 6 cuirassiers against 3 muskets in a city. as they are immune to 1st strikes, those 3 muskets will defend against the 1st three ok but then be so weakend that they won't be able to cope with the next 3, and the city gets taken.
the grenadiers with formation will be on a par with them but the chances are you'll get military tradition long before they'll get military science. and if they don't have formation then the bonus cuirassiers get against gunpowder units + the combat 1 & 2 you'll get with stables will make grenadiers ineffective.
the trick is to use cuirassiers either to support a sod with seige weapons and muskets to take a city quickly, or open up another front to stretch their forces.
as a defensive unit, the two movement range and flank attack against cannons means you can quickly decimate any enemy sod before they have the chance to take your cities.