Lineage Plasma Cannon (Apocalypse)
The Lineage Plasma Cannon is quite a powerful yet compact weapon, and it can fit on most vehicles, including the Hoverbike and the Phoenix. This weapon is fueled by Elerium and is probably the most cost-effective Elerium-powered weapon. It is a heavier equivalent of the Lancer 7000 Laser Gun for arming aircraft.
While a Hawk Air Warrior equipped with three Lineage Cannons can prove sufficiently powerful to take down a mid-tier alien craft, its indisputable biggest weakness is its appalling accuracy. And with its impressive range it tends to start firing long before it has a chance of hitting (the enemy at least). It will regularly succeed in doing ten times more damage to the city than the aliens if not handled carefully.
The other option is to only use them up close. The procedure used to conserve the use of the Stasis Bomb Launcher can be used here: disable the hardpoints mounted with Lineage Cannons, get in close, then re-enable the cannons and hit the UFO at close range.