Move aside Lightning Gun the new king of weapons is here!
Comparing it directly to Lightning Gun, you save two resources, get one more ammo and do one more damage. Note the damage is multiplicative with the ammo so you get potentially 16 damage to Lightning Gun's 9.
The first drawback is the body slot. Notably this rules out Bandolier. Thus a flame wielding Guardian will have to fully commit to exclusively using the Flamethrower by using Venturer, Extra Ammunition and Custom Ammunition. Contraband is also interesting if it is available to you.
The other disadvantage is you must be engaged with the enemy. Normally you do not want to risk shooting your friends with your high powered guns so you would want to engage anyways but this weapon actively requires you to be engaged. This may mean a loss of an action for a harried investigator. Also you may want to engage multiple enemies so its a good idea to bring Taunt along as well to avoid attacks of opportunity.
It still has one less fight then Lightning Gun but both are usually sufficient to guarantee success on all but the tentacle and the super negative token you get on Hard and Expert.
Overall I think despite these weaknesses this is by far the most efficient purifier of xenos scum, err I mean mythos monsters available today.
The M1918 BAR is also a very fun weapon and may become more efficient against very low health enemies, but there are too few of those around to compete.