Killstreaks were nerfed in Black Ops, and even moreso in MW3 (especially airstrikes). It's a good thing they did it too IMO. Makes the game more balanced, and CoD is always lacking in balance. The more the merrier.
Assault really isn't very effective except in FFAs, but it can be effective if ya got reliable team mates also running assault, though. Once y'all get an air support or two each, you can overwhelm the other team by a constant slew of air support. Replace the ones that fall one by one until your supply of lower air support is empty, then bring out the big guns that have a flare or two. This is especially effective if you can clear the skies of enemy AS prior to the onslaught and hold complete aerial dominance for a time.
Also, you don't just throw down a SAM, ya wait for air support you really want gone to show up 'n then deploy it. Use it like a Jav in a third weapon slot, not like a Sentry. Gotta drop 'em in open locations too so they don't just shoot at obstacles. If you do just drop it down, ya generally need to defend your turret. At the very least, you can nab a kill or two on the dudes lookin' to take out your SAM. Try keepin' your SAMs away from enemy lines so they ain't knifed as quickly too.