Note: thoughts are based on Advanced Random Map Generator with "very strong creatures" map setting.
Inferno units have low HP. Any damaged caused is always a casualty even in late game.
On the other hand Fortress can withstand considerable damage with minimum or no losses if utilized properly.
I have played both many times and actually Inferno are trickier to pull as they do not have shooters and their units are exposed to archers and magicians early to mid game.
I have a friend who is not that good at Heroes and he prefers Fortress faction because army consistency is easy to maintain during the battles.
Otherwise there were numerous Inferno vs Fortress battles where I have played from either side on equal conditions and results were pretty even which means both factions offer the ability for equal development. I believe the best way to be convinced is if we take this discussion into concrete problems and probably deliver a strategy to cope with it.
Here are some early game strategies with Fortress that help the faction kill monsters easily:
1. Might hero + War Machines + Tent and/or Balistta. Upgrade Tier 1 and tier 2 for even better results (get the Harpooner upgrade for greater damage output). Karli is a very good pick!
2. Magic hero + Destruction - any skills that improve damage output and mana attrition are good (sorcery, occultism)
3. Balanced hero - Light + hope for regeneration. Basically this class suffer the most casualties early game but has the strongest late game due to availability of runes and strong magic which the other 2 classes do not share.
- Against Melee:
Army: Defenders + Harpooners
Tactics: Use harpooners, hero and ballista to flank the enemy. Protect harpooners with defenders (5 stacks with 1 defender, 1 stack with the rest).
- Against shooters/magic dealers:
Army1: (enemy is average or weak damage dealer): Defenders + Harpooners.
Army2: (enemy deals good damage (master hunter?)): Defenders.
Tactics: if you go with defenders just hit defend and attack with hero, balista. Dont mind defender casualties as you get them in double population because of the Tier 1 dwelling.
Personally I dont attack ranged units except if it is not necessary (guarding important mine or staying on a way for another land) or I can overwhelm them before they cause havoc.
It is important to know that on a "very strong" difficulty, what you should do on week 1 is get ore + wood, get all unguarded tier 1,2 and 3 dwellings, get all free or poorly guarded resources in the zone. Your hero army is way too weak to conquer any mines this is what you try to do on week 2 (and probably 3 if needed). Somewhere between week 4 and 5 is the time to get out of the zone but this depends on many things (map template, how many casualties you had to suffer, unlike ToTe maps here casualties are unavoidable at some point).