Heroes really viable instead of a 6 Man Elite Troop?

It depends a great deal.  I've had heroes that could take on huge, powerful enemy armies or even a dragon single-handed.  I've gotten Ascian, a hero stalker you can get from a quest, whom has good damage with maul and can transport through several enemy troops in a line, causing heavy damage to each.  Its a matter of equipment and promotions normal trained troops (not counting Henchmen) don't have access to.

There's also the issue that ... if you had a hero versus a troop that, if they had equal total HP and damage output, the Hero would come out on top because as the troop takes damage, eventually the individuals in the troop will die, reducing its damage output whereas the hero will keep doling out its initial damage to the end.

Depends on the hero class and specialties, but overall heroes really tend to do better when leading an army stack and use their abilities to make the army stronger/compensate for weaknesses.

Only when a band of pure heroes are uber decked out in rare equipment and complimenting skills  do I think they stand a chance against a stack of plate-armored maulers.

in my experience, some heroes can still be very viable even that late in the game. their main strength is that they are viable much much earlier, though. with a few decent heroes, you can do pretty impressive stuff a long time before you can even train semi decent troops.

i tend to rely heavily on heroes early on. it's not uncommon for me to clear out strong and even dangerous monster lairs with my sovereign, the first 1-2 champions and some 3 man groups of spearmen (or other low-tech units) within the first 100 turns of the game (actually even killed a dragon once on tun 110 or something with just three champs and three 4 man frost staff units - didn't have anything better at that time)

the heroes that are still useful/good are typically mages/commanders; to some extent also defenders. assassins and warriors depend a great deal on what equipment you pick up. a high level warrior with a sick 40 attack weapon and "decimate" can slaughter companies of trained troops, for example. 

It depends on whether you are just talking about a warrior hero or heroes in general. A high-init mage hero could kite almost any high-level troop (unless they had that automatic first-strike, then they may get a one-shot win) and destroy it with high-powered spells. A single mage hero may be able to take out an army of elite units, with spells like dirge of ceresa or fireball.

A warrior hero is far worse off than a mage hero in almost every scenario, with the exception of when the high-level troop has the automatic first-strike.

the "Impulsive" trait was changed from  "automatic first strike" to +2 initiative at some point during the beta.

Mage heroes really compliment on another. Provided that mana is not scarce mage heroes can easily annihilate even top tier late game armies. The best part is that they don't even need fancy equipment to accomplish that, just some consistent and faithful training along the spell damage/spell mastery line + maximizing their magic schools. 

Which is good because there isn't any.

Yes Mages are useful from beginning to the end of the game. If the AI was smarter is could design units to resist your spells but that's beyond it so nuking armies is pretty easy.

I like commanders as well. The accuracy bonuses are worthless as everything gives like + 20 accuracy, but +3 initiative for an army is priceless. With command on top of that its more than worth it to have a high level commander in a late game army. I haven't used battle cry in a while but it was pretty broken.