Adding a heavy option to a unit such as a lancer or a rune golem , does this add any dice to the unit's attacks ?

I was told the golem adds dice to the unit , but the upgrade card does not say this, it just makes gives the unit Brutal (blue runes ?)

If so , is it worth adding heavy upgrades to units ?

The siege unit single figure upgrades don't add dice, they just grant special abilities like Brutal [Stable Runes] and spitting out blight tokens. They're also pretty tough units that take more hits to eliminate than a tray full of Spearmen or Reanimates, unless you get really lucky with Accuracy and Mortal hits.

Adding Kari or Ardus as a single figure upgrade to a unit does add dice to that units attacks as well as granting special abilities but they're pretty expensive upgrades. I haven't tried either out yet as an upgrade, but I'm more tempted to try Kari as an upgrade than Ardus.

The upgrade card will say what it adds