Naptha are very tricky to use, but very effective if you get it right.

Darwbacks: Short range, long loading/firing time, innacurate and pretty much random, chance of killing themselves or your own units.

Advantages: Devestatingly powerfull explosions, fast and manouverable, long enough range to fire over a unit, arc of fire, indisciminate killing regardless of health or armour of the target. Not particularly vunrable to archers. Reasonable moral.

The best use I have got out of them is in castle sieges, position them just inside the gate (not a for style gate, keep or higher) and when the enemy come and attack it, they will throw grenades through the gate, killing the enemy very well because of he enclosed space.

other than that, unless yo can get a good downhill position to fire from they are much better use to flank or rear attack with. Pin an enemy unit, the most expensive you can find, with something cheap, vanilla spearmen work well. (not peasants as they won;t last long eneough) move your naptha throwers round to the flank or rear and let loos a volley, remeber not to have them set to skirmish or they will never get close enough to fire.

The volley will kill loads of units, some of which will be yours, but the fear effect is often enough to make even high class knights flee, and your spearmen may well hold together because they get a moral boost for your killing of the enemy, not to mention you being on the flank and the moral boost for that. If the enemy didn;t rout, try again, if they did, start working your way up the line of battle.

They have enough armor ot to get wipe out by archers, so you cna safely ignore one or two of them firing at your naptha, and if you oly take along one unit it still has enough firepower to really change the course of the battle.

They are probably best used against royals, knights or the golden horde, all of which are igh armour/high defence units which take a hell of a lot of killing normally.