From Halopedia, the Halo wiki
The Banshee is an extremely agile aircraft. Under the control of an experienced pilot, it can be extraordinarily difficult to destroy, making it an easy way to penetrate enemy lines and harass enemy infantry. The Banshee's plasma cannons are highly efficient at killing individual soldiers, and a single fuel rod can eliminate an entire group of infantry. The boosting capability of the vehicle allows for quick entry and exit from combat situations, or to catch up with a particular target. Boosting and evasive abilities allow a pilot to outpace a homing projectile or slip away from opposing vehicles and small arms fire. By utilising aileron rolls and flips, the Banshee is able to avoid slow-moving projectiles at medium or long ranges, while also facilitating quick turns around obstacles or close threats. As a support aircraft, the Banshee is able to pin down enemy forces until ground forces can move in, or destroy light structures that may be obstructing ground forces from advancing.
The Banshee is lightly armored and, thus, vulnerable to high velocity rounds, explosives, anti-vehicle weaponry, and even small arms fire from human projectile weaponry. The Banshee's low flight ceiling makes it very vulnerable to weapons like the M41 Vulcan, which can easily punch through the armor of the Banshee and injure or kill the pilot. The Banshee is also vulnerable to EMPs, which can disable the craft temporarily.
Banshee variants that are not vacuum-sealed leave the pilot exposed from the rear, and the small gap between the cowl and fuselage can be exploited by a highly skilled sniper. Linda-058, one of the UNSC's best snipers, is known to have achieved such a shot.
The Banshee also cannot reverse unlike most aerial vehicles, forcing round trips to be made.
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The Banshee appears in most Halo Wars levels and can be used by players in skirmish mode who play as any one of the Covenant leaders. It can be built at the Summit with 200 resources, two population slots, and a tech level of 1. The Banshee has three available upgrades:
The Banshee is effective against most vehicles and some ground troops. It is capable of hovering like in Halo 3: ODST and is the fastest unit in Halo Wars. A common strategy is to create an army of about twenty Banshees and swarm the opposing army, especially if their forces are mainly composed of vehicles. Banshees are extremely vulnerable to M9 Wolverines or Type-29 Vampires.
Halo Wars 2
Banshees in Halo Wars 2 can be built by all Banished leaders at an Apex. Unupgraded Banshees perform well against vehicles, but only okay against infantry, aircraft, and structures. Once the Plasma Torpedo upgrade is researched Banshees will perform well against aircraft. In Blitz Banshees cost 50 power and Unstable Banshees cost 90 power. Banshees can be infected by Flood Seeder infectors.
Changes from Halo Wars to Halo Wars 2