Again, this is from experience with Britannia, but I imagine will hold true for other sorts:

General/Heavy Chariots:

Do not use them against:

Ordered/dense infantry formations, they will get stalled and killed (and usually your general is the first to go)

Spearmen/Pikemen especially, even in the back, they will turn around and muller you.

Do use them against:

Any kind of cavalry, especially in combination with cavalry of your own. Combined even with barbarian cavalry, heavy chariots will destroy even a full-strength Roman General retinue with few losses if it can get the charge in. I imagine this may be based on chariots being very good at knocking things down, infantry just pick themselves up, horses are crippled and are a casualty.

Loose, shaken light infantry (not spears/pikes!), chariots are pretty scarey and will put shaken infantry to flight especially coming from the rear or the side.

Routing troops, chariots cause very few casualties but they slow routers down enough for other troops to catch up, and they also ensure that routers _keep_ routing and do not reform.


Light Chariots:

These are the British uber-unit, even better than Horse Archers at the fire & flight and are therefore able to destroy staggering amounts of enemy taking very few losses of their own. I got bored with the Parthians because every battle was the same, I decided to go for the extreme west from the extreme east only to find exactly the same thing!

Do not use them against:

Archers, as they simply don't have enough firepower to win (12 bows vs 40). If you want to take down archers always concentrate all your fire on them and charge them whenever possible.

Light cavalry, they move too quickly to effectively fire and flight and if they get caught running they'll be for the chop. So instead just turn the chariots around and crash into them, it'll be far more effective.

Do use them against:

Anything else.

There are only two caveats to light chariots:
Watch the red line Skirmish mode works very well, up until you hit the red line (especially the corners) where the chariots go a bit beserk and try and skirmish away by going _through_ the enemy chasing them. This is especially bad if the battle has dispersed across the entire map, if the enemy lie in every direction (no matter how distant) the chariots will keep trying to skirmish back to the red line.

Count your archers Casualties heal by individual crewman, not by entire chariot. I have had three units of medium-sized gold experience light chariots go into a battle, I tried to put them on fire at will but the option was greyed out. I thought it was a bug until I looked at the chariots close up and realised that absolutely none of them had a second crewman.