The essential idea with disintegrate is that it's pretty much a point of no return for npcs. When you blast someone important like rolan or barcus into ash, they're gone for good - normal resurrection won't work and you'll miss out on their quests. Items, however, can still be looted and the disintegrated target will drop their equipped weapons as normal.

Something interesting to keep in mind is how your party reacts, since permanent kills tend to upset companions especially with friendly npcs like scratch or lia. The spell definitely changes how stories play out - like wiping out isobel completely changes the last light inn situation, and taking out nightsong affects both early and late game content. A few npcs like the hag have special protection against being turned to ash, but most story characters don't so you'll want to think carefully before using it on anyone who seems important.