How difficult was it to manufacture and arm a medieval army with incendiary weapons like Fire pots?

by Doncuneo

Fire pots seem effective against both formations of infantry and during sieges but they dont seem to be widely mentioned in most battles, What gives? Was oil and pitch rare or difficult to produce?

HermitCrabTuesday

A followup question, how would a soldier light the fuse/wick on a fire or oil pot if they were on a battlefield?

Another followup question, were flaming projectiles uses during sieges to try and burn structures inside a city?

SoloToplaneOnly

In Scandinavia, they are noted as the most effective weapon in ship combat and we have reports of them being used similarly to burning ships seen in Attila.