Yeah I mean that's definitely a fair point hehe.
The problem is that not-Power doesn't leave quite as much to work with as Power, at least in terms of gameplay mechanics. The Evil path does keep getting nudged towards the Good though, so if I had to pick I'd probably still go with giving more to the Evil Durge, like even more stuff, just to make sure that run remains satisfying. I think they could do more with the "Haunted" background, like where we have the ghosts trailing us around. Everything about the Durge is like just burning through companions left and right, so clearly the real boon is keeping your companions alive, but since there's a pretty good chance the player has already done that sort of thing with their Tav, Good Durge probably still needs something to ice it at the end.
Tadpole powers are much the same honestly. One thing I find frustrating in that case is that I don't really like permanent visuals taking over my avatar.
So having my eyes turn black and a bunch of veins bulging out, makes me want to avoid half-ilithid transformation, not because I don't want the power, but because the visual is forced on us. Allowing Volo to icepick you feels a bit different, since declining that one isn't as impactful. I think it wouldn't grate as much if the visual was only temporary, like a thing that we can't hide immediately after transforming using our powers, but which goes away if we long rest or splash some water on our face to snap out of it. I think this could be handwaved as dissembling magic. The same way Orin can wear whatever face, or Ethel can make herself appear harmless through deception. Anyway, as a general rule, spell-like vfx on the avatar should probably be temp visuals. Least with the Slayer form, when you transform back, it doesn't undo all that work you did in Char creation or at the mirror.
On that point, even the Mantle which you can usually wind up wearing with whatever sort of Durge, also ends up being an avatar visualization issue. I'm really not sure why they went with a cloak, since there's still a lot of clipping with those for weapons on the back. I'd probably be more amenable there with weapons on my belt, or a 'hide' mantle option. Those are modelling issues granted, but still, when the cape cuts your sword or staff in two, just makes me not want to wear it half the time, even if it's thematic and quite useful.