It's consistent in that it's easy enough to set it up to generate 20 power on Turn 1. And Turn 2/3 are only bad if you draw the other two of them consecutively (which with about 13 cards left in deck, is only a 1/156 shot of happening). In early rounds, it's usually rare you use a frost-setter, so having 4 of them in your deck (2 Hounds, Caranthir, Eredin/Dagon), so I don't really view the Frost Giants as must-mulls. Sure, sometimes planning requires you to skip your third mulligan before the start of the game, but that's because the general grip is fine (1x Crone, 0x Biting Frost).
If you go 2nd the first round, leading with Crones will almost always net you (a) 2 card advantage going into Round 2 or (b) force them to try to combo out with NR right away. Both are good situations going forward and obviously the thinning is great.
Tried that once, opponent happily took the win, passed round 2, then blew up everything with Kambi in round 3 while I looked on in disbelief. I mean yeah, it was the first time ever I played against Kambi, but after seeing it the first time one of the best defenses is to win round 1, and the crones-into-pass play doesn't do that. Then there was the time opponent played Foltest R1T1, and I could see no reason why he wouldn't pass, since his future plays are buffed while I'm down three silver cards, and he wouldn't even be taking a card disadvantage into later rounds.
I also don't see why you say it's usually rare you use a frost-setter in the early rounds, since it's extremely common I use one, possibly more, in round 1. They synergize with too many of my other cards (Drowner, Frost Giant, Jotunn, Imlerith, Wild Hunt Warrior ...). And if you don't have / use a frost-setter, why wouldn't the Frost Giants be must-mulls? They're just 5 power vanilla creatures otherwise. I've actually taken to playing Geralt first over Frost Giant if there's no frost on the board, since otherwise the Giant just dies to Triss Merigold, giving her a pseudo-17 power.
Finally it's only fair to compare the crones vs. 3 other silver cards. Competitors could be Nithral (hits a crucial breakpoint to kill 3/6/9 health minions, of which there are quite a few, e.g. Arachas), White Frost, Fire Elemental, Water Hag, and so on. Are the crones really worth the lost consistency + loss of these effects?
I don't doubt that the crones could be good, just not in the deck I have (or possibly my playstyle).