[Edi]

Different bonuses work different ways. Strength and hit points are a percentage of the original base values.

Heroic Toughness grants +50% hitpoints and keeps increasing, Heroic Strength does the same for Strength. MR, Defense, Precision, reinvigoration, Tough Skin and others all start at +1 and move upward by increments of one.

Therefore Heroic Strength and Heroic Toughness are especially good on big units like Niefel Jarls, Rakshasas, Fomorian Kings and other high impact units.

Funny thing is that if the unit's type changes, such as bringing back a mage as a mummy, the strength and hitpoint values are calculated as percentage points based on the base values of the mummy. Which can make for an instant thug when kitted out. I had one mummy with the hit point boost ability and he was at something like 100 hit points and once kitted out, slaughtered armies on his own.