It's Maggie and Adam who parallel the Watchers and Slayers. Maggie Walsh tried to harness the powers of demons, just like the Shadowmen did long ago - but Maggie came at the idea from a scientific/human angle, taking bits and pieces from demons and humans and putting them together like a quilt. This is in stark contrast to the Shadowmen, who decided to fight the demons on their own turf - they used magic to create the Slayer, and took the spirit of the demon to imbue a girl with super strength, leaving her with a mind of her own to make moral judgements, rather than Maggie’s ‘programming’. Of course both Adam and Buffy were supposed to be guided by a 'parent' - but where the Council was always a very strict and distant, seeing emotions as weakness, Maggie deliberately developed a strange, twisted mother/son relationship with Adam and the rest of the soldiers.
And because Maggie was the type of parent who simply refused to allow her "boys" to think for themselves, they rebelled and took her power (aka life) away from her. While Giles, the Council representative, is smart enough to know that Buffy simply can't be contained and allows her the freedom to make her own decisions which gives her the power to save the world over and over while Adam and Riley sink into nothingness. I mean Riley goes off to fight demons in Central America but that's hardly save the world material as far as we can tell and frankly do we really know what those demon eggs would have done anyway?