I frequently get Ruthless Disposals late (I got one 10th the other day!) and I consider it the best Black uncommon in Eldritch Moon. Obviously many people disagree or I wouldn't be getting it late. In defense of one of my pets cards of the format, the effect is incredible and the drawbacks are manageable.

Effect -- Kill their best two creatures, no questions asked. The only outs are Hexproof or a counter. That's one of the most potent effects you can print on a spell.

Drawbacks -- You're killing their two best creatures, so of course there are drawbacks. Most of the time you will discard an excess land, but it might also be a spell that has lost value such as a 2-drop drawn late. Madness is unlikely but possible. As for sacrificing a creature, it's not unusual to have a disposable creature lying around in the late game. You do have to be careful that you don't trade away your entire board and make it uncastable but similar to saving a bad card to discard, you can plan for it. 5 mana and sorcery speed are exactly what we expect to pay these days for unconditional removal so I don't even consider those drawbacks.

As a first pick, I don't consider this even close. Grim Flayer is a gold card that in many board states will be Grizzly Bears. Sure it has far more potential than Grizzly Bears but if you don't hit Delirium, what's your plan to get value out of it?