Zombies

No kidding, I just asked Google if one can effectively use a flamethrower from a helo: Google said

Sure you can, though for killing zombies it would be worse than useless, especially in the case of World War Z (film) zombies.

Zombies are generally considered quite difficult to kill and in most lores require the destruction or incapacitation of the brain. Fire doesn't do this, at least not in any reasonably quick timeframe. In World War Z (film) the zombies can run very fast, can jump, and take no notice of being shot or on fire. All setting it on fire does is mean you're now fighting burning zombies who can set buildings on fire, which means that even if you can fight them off, you'd still have to evacuate your settlement.

In humans, setting them on fire tends to distract them from other events with the whole "being on fire" situation occupying them most pressingly (citation needed) . This is why using flamethrowers against humans works rather well because not only will it incapacitate them, but it also spreads fear among other soldiers. After all, who really wants to be burnt alive?

It is this difference in tactics that is important to consider whilst fighting zombies rather than humans, as any tactic made to impact morale or pin enemies down is ineffective against zombies. Failure to consider this is the main reason why in World War Z (Book) the US lost the Battle of Yonkers