Ork choppa squads usually come in groups of 20 and above, not including nobs, if your opponent is smart. They have very low ability when it comes to pretty much anything ranged, so their duty is to run across the battle field, being torn apart by enemy fire but still having enough to cut up the enemy once they reach close combat range.
One on one, Orks are not powerful. However, in their hordes, if they reach the enemy gunline, they are probably the most dangerous units in the game.
Anti armour is a bit of a weak point for Orks, though. They have a few vehicles that are good at anti armour but they are made of paper. Usually you'll have a few tankbusta squads or nobs with power klaws and about 15 ork choppas protecting him. Power klaws will probably be the bane of your existance if you field mostly vehicles. The nicest thing about the rules for an ork player is the person who's squad it is chooses who dies when they are attacked. This means that if you had, say, 20 orks and 1 nob with a powerklaw running toward your tanks, chances are you're going to have to kill all 20 orks to have a chance to take out the power klaw nob.
Ork vehicles have horrid armour, and horrid aiming. But their quality is in their number, and the amount of times they can fire. A few squadrons of Ork deffkoptas with rokkits can take out pretty much any vehicle if you let it.
Ofcourse this depends on what kind of clan you're fighting against. The Goffs take a very IG approach and simply throw wave after wave of Ork green tide at you until there is simply nothing left. Clans like Evil Sunz, which is what I play, have a lot of very fast and very mobile attack vehicles like Trukks, stocked to the brim with boyz and they speed into combat and hassle troops before driving away again and coming round for the second attack.
Sorry if my post is confusing. I am tired.