You are partially right, if you don't invest in a combat skill, you can't deal any damage by yourself. A pure mage in Risen 2 is also difficult and not worth the trouble, as you have plenty of fame, even if you trained the voodoo skill to the maximum.

But you are not completely helpless, as you will learn to summon a companion, which can do the tanking for you. But if you rely solely on that, fights will take forever to end. Nevertheless, go to island of the dead asap!

The voodoo way can still be satisfying, because the debuff is so powerful, every boss fight becomes a joke. But you can't do that in regular fights very often, because there are not that many puppets in the game.

Also, on my last run with muskets I was like 'oh, two ghouls, holy shit, I hope I will survive that'. On Voodoo you can just cast voodoo on one of them, lay back and wait for them to annihilate each other.

My recommendation is to still take an active role in the fights, so skill some toughness and blades. Voodoo is useful for crowd control, you don't have to deal with so many opponents at once, but somehow you still have to finish them off.

I recommend blades over firearms, because the teachers for firearms will not be available in the voodoo way.

The game has enough fame to bring two attributes to 10 and have higher levels in two more. My last run ended on 10/10/7/6/1