The FAV attacks multiple targets within range at once.
The FAV will only use one stream to burn a target even if the target is the only one on the battlefield in range.
FAVs use one stream of flames per target (on level 8 they have 4 maximum streams of flames). So when they're facing one target, the FAV shoots one stream of flame, doing its DPS. When there are 2 targets, it will shoot 2 streams of flames each with the DPS given. But if the 2 targets are too close together, they get the splash damage from the other flame too, effectively doubling the damage it receives. So if 4 or more targets are bunched up, all the targets will receive 4 times the DPS of a FAV. That's why the FAV does more DPS to bunched up units than to singled out units. So spread out your units and they will only get the DPS of the FAV once, making the FAV less effective in total.
Unlike most units that you target directly where desired, FAV perform more effectively on "strafing" style attack runs. That is to say instead of hitting the target directly, move FAV in a straight line past desired target allowing them to burn it to ashes with some health left, hopefully
The FAV houses a deadly cargo of Flamethrowers, The FAV's passengers spray fire even while it's on the move, and continue the assault upon the vehicles destruction
— In-game description
"Liquid thorium enhances the fire launched by the passengers of the Flame Assault Vehicle. Enemies caught in the crossfire are set ablaze."