1) The Ravers: No ill effect as far as we could tell. It only made them stronger.
The ravers may be in a position where simply being stronger does not help them. Unless they have the strength to challenge Foul, they are merely his servants. And they certainly were put, as a result of their enhanced power, in places where they seemed to most regularly experience "death" throughout the series. I suppose it also allowed them to do much more harm. I don't know the ravers very well, and I don't know if any kind of death displeases them, or if harming others is worth more to them than displeasure or pain. Maybe those eastern terms that describe them imply a kind of detachment that allows them to delight in the pain of others and themselves.
2) The 3 Haruchai survivors of the mission to the giants: It strengthened negative traits in their personality but didn't harm them physically as far as we can tell. Their maimed hands were Foul's handiwork
But the stone seems to have broken those three people more than they would have been by being maimed. Regardless of whether the harm was physical, I think the stone alone was far more potent in harming the Haruchai than say, a pack of kresh. It made them suffer, and led them to a particularly awful path towards death that is neither long enough to allow respite nor mercifully short. "Exactly wrong" instead of "exactly right."
I think Covenant felt he harmed Bannor when he argued that the Vow was compromised, but I think that treatment of Bannor and the other Bloodguard was far less harmful than what the stone did. I also don't think of it as a kindness like when Covenant released the ranyhyn. The ranyhyn were bound against their will, but I think it's clear that the Haruchai's wish for the land to be preserved outlasted their desire to preserve it - that's why that latter desire came back, and the Haruchai came back. Yet I'm not sure the Haruchai were as doomed to die by the vow as the ranyhyn were had they kept to Covenant's command. That's just an attempt to explain why I see Covenant as having harmed the bloodguard in that instance, and I think I've already placed the stone as something far more harmful in relation to it...Of course the stone also had valence in disgracing the oath of the Haruchai.
One of the giant-ravers - the one who Saltheart and Covenant met in the creche - wasn't even alive anymore. I don't really know whether that shade was possessed by a raver, so I don't know if the persistent malignity of his shade can be attributed to the raver or the stone or the tenacity of the giants.
3) The residents of Stonemight village (in the Wounded Land). Longest owners of any piece of the stone. They seemed physically stronger and quicker though no match for anyone with true powers (like Clave members) but again spiritually ugly. Also the region they found the stone at is barren.
So the stone makes (semi-)nice people less nice and makes un-nice ones stronger and more effective. You don't need to be The Foul to be on the good side of it.
Hmmm...in the long run, in the grand plan that probably goes beyond Foul's control even if it will serve his interest, Foul's capability for destruction was obviously enhanced. He did suffer a "critical upset," however. Moreover, simply because Foul and the stone are on the same side, does not mean the stone did any good for foul. Maybe Foul doesn't know what's good for him, and all this maker-work is bad for him. And I guess you could say that the stone took a major loss when Covenant took it on.
I don't know about those folk in TWL, because it's been too long. But it doesn't seem like "stronger" means effective. We haven't really seen those "effective" people achieve any goals that seem to be worthwhile for them or anyone else. Who knows. Maybe they're happy enough in the moment...but the bloodguard, at least, don't come across that way. Nor do the ravers, who seem to betray a deepseated fear of Foul - serving him is their best and only option, but maybe not a very good one aside from that.