Well this is interesting. I actually haven't bought it yet, I'm waiting a week to actually click buy. I did get the Ravenloft Bestiary by Leonaru from DM's Guild which I think is excellent and which to my delight had several small fey in it that I can use. So that has opened up the choice again a little. One thing I'm particularly interested in is the Hag stuff from Volo and currently that's probably its strongest draw for me. Is it really that good? I have heard people raving about it. Can somebody give me just the gist of the new background stuff for them? The hag stuff is now likely the deciding factor between the books for me so is it good? Does it go into more detail about their fey origins or where they come from?
The gist of it is that hags in Volo's are less like wandering sacks of HP and more like Baba Yaga, Ursula the Sea Witch, and the witch who tried to eat Hansel and Gretel. They are unpredictable and full of weird magic (the DM is encouraged to give them single-use magic items that mimic CR-appropriate spells, like a bottle of wasps that surrounds her and stitches up her wounds with their stingers (acting as a Cure Wounds II). They reproduce by snatching and eating human infants, which causes them to later give birth to a child who will turn into a hag on her thirteenth birthday. They love to set up shop near human settlements and wait for humans to come to them trying to secretly make bargains: think Monkey's Paw stuff here. There's a lot of discussion (a couple of pages) on the nature of the bargains they like to make and how they enjoy corrupting whole communities. Solitary but social, they avoid the company of their peers (unless it's necessary to form a coven in pursuit of a greater goal) but enjoy hearing gossip about them. There's a hag hierarchy, with "aunties" and "grandmothers" who are more powerful than normal hags--some of them even have lair actions although I didn't see any guidance on stats for them.
Hags are long-lived, have inverted standards of beauty (perceive warts and bruises as beautiful, and find regular features and even white teeth unpleasant, etc.) that make them outcasts in fey courts, always have an escape plan, and are loath to engage in physical combat (because it's risky) unless absolutely necessary.
I'm left with a definite feeling that (1) hags, if played according to Volo's standards, should be used almost more as plot devices than "encounters", and certainly not as combat encounters; the presence of a hag in an adventure should imply secretive villagers, corrupted animal monstrosities, weird gingerbread creatures, a troll servant whom she rides around as a mount, and an escape route for the hag if things go poorly; (2) standard MM hags are best used at very low levels or against small parties. A 6th level PC with three companions visiting a hag to bargain for the return of a young man's wits will not be properly terrified of the possibility that she will bite off his fingers with her iron teeth--if she tries, he and his companions would simply nova her into oblivion. At minimum she should insist on seeing him alone, but there also needs to be something preventing them from simply cutting their way into her abode and forcing her at knifepoint to give the wits back, and that works better if the hag is CR 9+ with everything that implies (a "weird magic" bound fire elemental roaring in her hearth, the ability for her hut to simply get up and run away from the player characters as fast as a galloping horse, the fact that the player characters would have to cut their way through an army of animated wax sculptures which turn (back?) into dead children when slain, the fact that the hag has leverage over the villagers who hope for the return of their missing children someday when their term of service as wax sculptures is up).