Basically, they're somewhat less good than a low-level Hero or Henchman with no magic paths would be at the stage of the game you can train them. I tend to find the warrior/assassin/defenders pretty underwhelming at that point. They can be good for a few special purposes, if you're willing to babysit them for a few levels, and you don't already have another hero that can perform that special need. They start with a higher base initiative than regular units. They don't have the faction's blood effects (good or bad).
As Warriors for raw damage dealers, they will generally under-perform since their maximum damage is restricted by the largest damage weapon you can provide them (Doom Maul) + Strength + Lethal + damage amulets + weapon trait bonus if not swords + any other trait mods - vs a regular trained unit which "just" has a X6 multiplier. Usually the X6 is much bigger.
As Defenders, they can be more or less as good as heroes, as the Defender abilities are less equipment reliant. Most people don't favor Defenders, though.
As Bow Assassins they could potentially be OK as anti-hero snipers taking the Executioner path (since enemy heroes also have low hitpoints) and the Ignys bow is about as good as any you're likely to find as loot. However, they're strictly inferior to a mage hero in that regard, who also won't be handicapped by carrying a big -init weapon.
As back-row field Commanders, taking the +accuracy/init/XP line, though, they can potentially shine (if you can keep them from being sniped - trade off of armor vs. initiative.)
