Just gonna pop my head in real quick in relation to incendiary rounds.
For a weapon like the flamer or Banshee, Greased is usually a better option.
Not only do you get more ammo, but it doesn't travel slower, and it shoots faster.
This helps the flamer because instead of raising ignition chance, you raise the amount of particles per second.
Not only are the Banshee and Hades more satisfying to use with Greased, but, once again, you're getting more ammo faster, so one shot missed is more forgiving.
In practice, use greased for weapons already notorious for starting fires, and incendiary to give a hot surprise on guns that normally wouldn't start them.