Yeah, that's a fair point - things like Knights are the same even in different armies.
Here's something else - only one stand, compared with the cannons having two...
I wonder - is this kind of unit costed differently in any of the experimental/fan armies?
The further away from the core lists you get, the cheaper / better "Stone Throwers" get.
Well the "Doom Diver" from the Goblin list is an 80 point catapult unit with 3 attacks, it doesn't have any special effect on the targets armour save but it does cause confusion on a 4-6... it's also a 2 stand unit which makes it an absolute bargain! Not sure if it's a fair comparison though as the low price could be to compensate for the armies very weak troops and low command?
The Doom Diver is a classic example. If the Doom Diver is really worth only 80pts, then the Rock Lobber cannot be priced at 75pts. Confusion on a 4+, 6 attacks per round, the target getting an armour save is not trivial, however most stuff in the game is lightly armoured.
Wounds per round of shooting AV0 3, AV6 2.5, AV5 2, AV4 1.5.
Chance of confusion per round of shooting AV0 150%, Av6 125%, AV5 100%, AV4 75%.
Compare that with the 1.5 wounds and 25% chance of confusion from the Rock Lobber and the problem is clear. Even vs AV4 the Doom Diver delivers the same number of wounds and 3x the chance of confusion.