Isengard has the best common troop type in Uruk-Hai warriors/scouts. I'd say they rank as "semi-elite" units. They have weapons specifically designed to counter cavalry and will be pitted most often against the Rohirrim. Berserkers add even more punch to their ranks. Crossbows are just plain nasty, as the most powerful, if immobile archers in the game. If you somehow find a niche these troops can't fill, then they can also ally in Orc Warriors or either type of Dunlending. For example, Uruks have no cavalry of their own but can easily add Warg Riders which feature the additional bonus of 'fighting' steeds. The Uruks can demolish defensive positions with a variety of siege weapons.
Isengard has an abundance of named heroes, and of course, always has he option of bringing out the White Wizard. There's no reason for them ever be outmatched by another faction's heroes (that is, unless the Fellowship's in play).
All in all, ISENGARD ROCKS!
...but the beauty of SBG, is any other army can beat them on any given day.