Quote Originally Posted by Marduk View Post
flaming missiles scare troops and you can light up siege equipment with them ... (and they are probably less accurate??)
They are recommended against units that can run amok (elephants, scythed chariots), because the fear penalty will cause them to run amok sooner. However, the first few rounds should probably be normal arrows, as you do need to kill a few before morale becomes low enough that the flaming arrows can trigger amok.

There is also a persistent myth that flaming arrows are better against non-scythed (i.e. Celtic) chariots. This is not true: these have high morale and cannot run amok, so you better use normal arrows against them unless they are very close to routing.