On just about any Internet forum concerning guns and/or survival, you will find at least one person who spreads the myth that the AR-15 design rifle is unreliable, while praising the AK design to the high heavens, often with claims that you cannot cause a failure in an AK-47. Most of these people blame the "problem" on the direct gas impingement system of the AR, which is why so many gas-piston uppers have been introduced for the AR design. Piston proponents point to these new uppers and say "See? Those wouldn't exist if there wasn't a problem!" Actually, they exist for one reason and one reason only: because a market has been created by all the hype. Add five bucks worth of parts to an upper and sell it for three hundred bucks more than a standard upper? Of course somebody is gonna produce it!
Another theoretical shortcoming of the AR design, although not generally known by the typical Internet commando, has at least a grain of truth to it. It goes like this: When the bolt contacts the top round in the magazine and starts pushing it forward, resistance causes the cam pin to rotate, pressing against the side wall of the slot in the receiver in which it rides. Theoretically, the presence of dirt and grit can increase friction here enough to cause a failure to feed.
If that's happening, you need to replace your buffer spring.
This video demonstrates the reliability of a basic AR. Although the handguards are different, it is a standard carbine-length, direct-impingement (non-piston) gas system. In the course of firing 3000 rounds through this upper, they seem to be using two lower receivers: the semi-auto only lower used in most of the test, and a full-auto receiver used for part of the test. Watch the amount of sand and dirt they pour directly into the action, making sure to get plenty into the area that supposedly causes a problem with the cam pin.
Look, anything can be made to fail, especially an autoloader. I don't care how reliable you think an AK is, it can't hold a candle to, for example, a Mauser '98; nor can a Glock or any other autoloading pistol match the reliability of an old-model Ruger Blackhawk. In the end, it is training and familiarity with your chosen weapon that makes all the difference. If you think being armed with an AK-47 automatically (pun intended) guarantees that you would win an altercation with a guy who is armed with a break-open single shot shotgun, you may just get the surprise of your life.