Speaking as one who has neither cavalry nor artillery, and probably never will, it appears to me that cavs are OP, and artillery are UP. I’ll discuss cavalry here, having already discussed artillery elsewhere.
When I’ve fought alongside (non-dwarf) cavalry, they typically go charging into a sea of enemies, heedless of any danger, often winning against three or four enemy units at once. This should not be.
Historically, cavalry are devastating against archers and undisciplined troops, but virtually impotent against well-disciplined infantry, particularly spearmen. In game terms, this could be simulated by giving them attack and defense scores comparable to HI (no greater), but dividing both by two when confronting LI or HI head-on (i.e. with no flank bonus).
In reality, there are as many different kinds of cavalry as there are infantry. For game terms, it would make sense to distinguish light cavalry, heavy cavalry, and ranged-attack cavalry (like the Mongols or American Indians). That doesn’t necessarily mean every race would have access to all three. Elves might have only light and ranged cavalry; dwarves might have only heavy cavalry; and so on. Probably only humans would have all three. Light cavalry could be introduced much earlier in the technology progression, perhaps on a par with heavy infantry, or even archers.