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.

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. 

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

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.