The thing is, most people don't stack hoplites or chariots. In my year and a half of playing this game, I've rarely, if ever, seen anyone stack hoplites or chariots in a city. You usually see 1k swordsman 1k archers and maybe 800 hoplites. While 800 hoplites is close to 1,000, in order to have an effective blunt defense you need 2k hoplites (1 swordsman, 1 archer, 2 hoplites is a ratio that puts your defenses as close to even as possible). A few horseman nukes will have no problem clearing that out. I use hoplite and slinger nukes as fodder for catapults, then bring in the horseman to finish the rest off. They always slaughter incredible amounts of troops. The build times aren't that long. If you min/max your island resources for your nukes, and research conscription and trainer, you can pop out a nuke, without gold (just a bit of favor) in 2-3 days on speed 2. I always build up my barracks to 30 for horse nukes anyway, so build speeds are irrelevant.
I think people look at the numbers of simulated battlefields too much, and don't look at the actual circumstances. Next time you have a city under siege, look at the spy reports. Add up the defenses. The weak point will most likely be blunt. Granted the attack per population is quite a bit lower than slingers, but if you had equal numbers of swords, archers and hoplites (I dislike chariots, they aren't bad, but not for me, personally), say 100 of each, you'd have:
| Blunt | Sharp | Ranged |
Swordsman | 14 | 8 | 30 |
Archer | 6 | 25 | 12 |
Hoplite | 18 | 12 | 7 |
| | | |
Count | Total Blunt | Total Sharp | Total Ranged |
100 | 3800 | 4500 | 4900 |
Blunt is the weakpoint in the defense at this point. Granted, if you used 100 pop's worth of chariots instead of hoplites, it would look like this:
| Blunt | Sharp | Ranged |
Swordsman | 14 | 8 | 30 |
Archer | 6 | 25 | 12 |
Chariot (Divided by pop) | 19 | 4 | 14 |
| | | |
Count | Total Blunt | Total Sharp | Total Ranged |
100 | 3900 | 3700 | 5600 |
Now, you have weaker sharp defense, but the blunt defense is still quite low, and has only gone up 100 points.
So you have 300 population of defense, so let's look at 300 population of horseman:
55 blunt damage * 100 = 5500
This absolutely slaughters the above troops. If the target had appropriate defensive ratios and the correct numbers in their city, they could close that gap, and then if the numbers are even, you aim for the most damage per population, which goes back to the slinger.
It all comes down to attacking the weakness, and taking advantage of the situation. If you face an alliance that always has tons of chariots or hoplites defending the city, you need to do the math to figure out the best course of action. Attacking swordsman nukes with slingers is foolish, and attacking chariot/hoplite nukes with horseman is madness.
tl;dr
Horseman are fine the way they are. Unless you plan to make all units have the same values per population, and make the game less about strategy and more about brute force, they are appropriately strong.