Heroes 5.5 WarGame Edition - Tactical Edition.

Artillery: Before each battle, 10% of enemy troops are lost (rounded down). During a siege, the allied catapult fires 2 times per turn and is controlled by the player. The supply wagon is restored after the battle.
Sulfur Rain: Before each battle, a total of 20% of enemy troops are lost (rounded down). During a siege, the allied catapult fires 3 times per turn.

- Isn't that a bit too much...

I agree, it looks very strong (but with 5/10%, it's not attractive at all). However, it's worth considering that the base itself (engineering) is weak and doesn't combine well with these effects. In essence, it gets 20% destruction and symbolic units (without any perks) at the cost of 4 perks. Also, rounding is applied, which means that tier 6-7 units are not particularly affected by artillery fire (and the player can counter this by splitting top-tier units into stacks of 4 or 9 creatures). When capturing banks (tombs, etc.), the bonus will also not work, which reduces the value of the perk in breaching defenses.
So, I would rate the 10/20% artillery fire at around 6/14% due to rounding – these numbers are still within balance. Do you think that an upgraded rapid-fire ballista will deal less damage per battle? :) I didn't mention it, but the ballista will be powerful enough that its branch will be strong even without firearms.

Interestingly, why do they mention the "troop gathering + leadership" combination as being effective (although it also requires leveling up to level 3 and charisma), but don't say anything about Guildmaster? At levels 8/18, this provides +1000/2000 gold per day and a -2 penalty to the enemy hero's morale upon encounter (bribing). I think this is definitely not worse. With that kind of money, it's worth adding Charisma to Troop Gathering + Guildmaster, allowing you to buy off 20% of each neutral army. The Reaper/Cultist/Necromancer, with skilled leadership and Guildmaster, will reduce the enemy hero's morale by as much as 3+2 = 5 points.

In reality, many of the hero perks look really cool, just like many of the creature perks in this mod (which, moreover, have shown good balance in duels). And this personally inspires me not to switch to weaker perks. I'm strongly annoyed by minor and symbolic bonuses; my soul craves grandeur.