An ammo should reflect the description and not have hidden or special effects on specific guns. Visual effects can be an exception but that requires Muse work.
Increased aoe would have a small effect on hades and banshee as with burst. It would mostly affect larger aoe guns like hwatcha and mine launcher. The mine would be too powerful with incendiary and it's already great. We should avoid buffing the mine launcher, although the below proposal is a mine buff.
Increasing the secondary damage fire chance would be a bigger buff to guns (except flame) than a higher primary chance. For example +10% primary +30% secondary fire chance. Incendiary can afford to be good on the flamethrower but I'd put a max limit of 10% more potent than greased.
Separating fire chance is a neat idea that I like, but it needs a supplement. I'm convinced that the simplest and balanced way is to change the rof reduction from -30% to -10 or 15%. The low dps makes incendiary unusable on most guns and this would make it work on those with potential, and good on those that it already works on. Examples of potential are gat, lumber, hades, taur, and hflak.