Not if the banshee and the pilot are at full health. I've had many a staredown when I was in a Scorpion and some other guy was doing the banshee bombing thing. Also, it's hard to pull off firing 2 whole clips of DMR ammo at a vehicle that can not only evade, but can shoot and evade so quickly that it is nearly impossible to get an actual lock on with it. Also, good luck pulling off a direct hit from a scorpion against a "flipper". And if it's so easy to destroy, then why are there people who go +30 in 20+ games in a row using it?
You don't even have to have that much experience in a banshee to be able to get at least a spree with it. The Scorpion is vulnerable to every single weapon on the map (literally, even your fist). The Banshee, on the other hand, isn't. Plasma Launchers and Rockets will only land a hit against someone who doesn't know how to use a banshee, small arms fire will only attract it's attention, and the only two weapons that are effective against someone who has any experience in a banshee are the Sniper Rifle and perhaps a lucky shot with the Spartan Laser.
Oh, and if you get REALLY lucky, you can take it down with a donkey punch. However, you still end up dead in that scenario.
The fact of the matter is the Banshee is the hardest to kill and easiest to kill with vehicle on Halo Reach. That makes it OP.
Only if they added some extra armor plating to all of the other vehicles as well, and took away the banshees unlimited evades. I mean seriously, it's a bit of a joke that you can kill a Scorpion tank (a vehicle that is SUPPOSED to be small arms resistant) with the smallest arms in the game (e.g. the magnum and plasma pistol).