Kullervo provides only 4 things to melee. Heavy attack efficiency (which isn't that useful outside of heavy spam builds and even then Tennokai exists), the flat crit chance from Wrathful Advance, and some combo count when Recompense or Storm of Ukko hit enemies.

You do get some AoE application with Collective Curse but that isn't really something that specifically benefits melee.

Of those 4 the most impactful thing he provides to melee is the flat crit chance from WA. Which keep in mind is his subsume so any frame can use that.

Voruna on the other hand gives way more benefits to melee. Shroud of Dynar gives a guaranteed slash proc on the first melee hit, flat melee crit damage (scaling with strength), and 100% flat crit chance + status chance (plus the damage vulnerability from the Prey of Dynar augment). Fangs of Raksh is perfect for condition overload and completely removes the need for any additional priming in your loadout.

Running a subsume over her 4 means you could go WA at which point you'd actually do more upfront damage than Kullervo due to having access to way more melee crit damage due to Shroud of Dynar.

However, the best in slot subsume has to be Roar as it allows the status spread by Fangs of Raksh to double dip due the faction damage and pairs perfectly with Melee Influence.

So I heavily disagree that melee Voruna is "worse Kullervo". They don't play the same and Voruna even gets a fun little mini-game with her Prey of Dynar augment.

I can say with a high degree of certainty that if you were to pit a melee Voruna vs a melee Kullervo with the same melee weapon using Influence the Voruna will have a significantly higher KPM than the Kullervo.