Sion Units?

Pretty much (although the multi-unit version the Undead faction can get by exploiting their Undying Curse trait are much nicer   )

They do make good governors though, so I end up building a lot as Empire.

Now Henchmen, on the other hand, are worth their own weight in gold...but that's kind of Altar's "thing".

You're aware you can outfit them with armor and items, right? Just edit the base Sion unit in the unit designer. Give them some nice equipment then have them fight easy battles until they've levelled a few times. Of course they aren't going to be good for much if you send them as level one to fight dragons but they can level fast if taken good care of.

My sion died to an city archer.

Which is possibly the worst unit in the game second only to the darklings.

That doesn't really answer my question, but OK. Next question, then: Are you aware regular heroes only have 20 health base? In that context the Sions starting at 10 doesn't seem so bad.

Don't send level 1 Sions to fight powerful enemies, same as you can't send ditto Henchmen to do so.

Not sure, Need to replay undeads and find out.

And plus I doubt a level 2 town protected by nothing but the militia kinda qualify as powerful

It qualifies as powerful if you don't put any armor on that level 1 Sion. Armor 0 is a death sentence against archers.

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.)