So regardless of the person, their talents or manner of death, exactly the same methods should work to resurrect them?
Ygerna's father was a necromancer, but her part in his research involved kidnapping children. She was at least moderately strong as a witch, but still only about 20 when she was beheaded (and then soul-forged to Damian, in a variation of that spell he had to come up with on the spot).
Sassan was older, and Laiken was doing much more advanced research. Whether she became suspicious of Laiken or not, she had all kinds of time to prepare for her own resurrection. Laiken may have even helped her with that, assuming it was done before he found out he needed her blood.
The ritual Laiken did to unseal the battle tower may have resulted in Sassan remaining as a ghost, or that may have been the usual anger / unresolved business method of ghost production. The candle trigger and windy grave may have been Laiken's method to keep her ghost away, but still available if he wanted to gloat or needed information, or something. Alternately, it could be a test Sassan set up, since there was no point standing around at her grave if there was no one around strong enough to help her get revenge on Laiken. Rather than being a 'plain simple ring', it was the key to complete the resurrection.
There was yet another method of resurrection in Divine Divinity (aside from the blessing ritual), where a necromancer made preparations, and then required some time to pass before skeleton servants would pull 4 levers, in order, to activate the mechanism that would bring him back. Though that method didn't quite work as planned, he did return alive rather than as a zombie / skeleton.
