Here's what I allowed for my Curse of Strahd game. Note that this was right after the Warlock/Wizard "issue" had been released. Anything after that hasn't been reviewed as much. Also, SCAG is not used, but Elemental Player's Guide is permitted but no one has picked anything more than a spell from it. The Ravenloft options didn't wow anyone, either.
Few racial variants (kinda). Based on the "humans are preferred" text in the adventure, I actually encouraged more human-looking characters. Human, half-elf, aasimar, and tieflings. Tieflings were reskinned back to the 3E almost-human appearance. All the PHB races (except drow) were actually available. I just made a point of there being in-game consequences for selecting those races. I did allow (Eberron) changelings, as well, but pointed out that, while they could pass for human easily enough, it would be extremely bad if they were discovered.
Classes/kits (assume PHB options, only listing UA extras):
Barbarian: Ancestral Guardian, Storm Herald
Bard: Glamour, Whispers
Cleric: Forge, Grave. Note: these were presented as granting martial weapon proficiency, instead of heavy armor because personal preference.
Druid: Twilight
Fighter: Arcane Archer (contender, but ultimately went with Ranger)
Monk: None offered. Probably would have looked harder, had I realized we were going to end up with a "China/Japan comes to Eastern Europe" instead of the more traditional setting.
Mystic: As written except no Far Realms.
Ranger: PHB version permitted, but latest UA rewrite promoted. Beast, Hunter, Deep Stalker, Horizon Walker.
Rogue: Scout.
Sorcerer: Favored Soul, Phoenix, Sea, Stone
Warlock: Hexblade, Raven Queen.
Wizard: None permitted.
Spells: Various. Starter Spells article all looks good and will probably see use as characters gain spells.
Feats: Would selectively allow racial feats. Would not use skill feats.
Downtime activities look nice, but haven't had any downtime since it was released.
Current party:
Aasimar Devotion Paladin
Aasimar Favored Soul Sorcerer
Half-elf Moon Druid
Human Open Hand Monk
Human Hunter Ranger (UA variant)
Previously used:
Prior UA Ranger (with 2d6 hit dice and Ambuscade). The 2d6 hit dice played well. Ambuscade was way overpowered and annoying. The character started as a Rogue and multiclassed into Ranger, so that may have been a factor in the Ambuscade being annoying. The player was not a true power-gamer (competent, not aggressive), so it was done for RP, not to game the numbers.
We also used action points for Eberron, but allowed PCs to swap for a Dragonmark, if they wanted (basically, limited selection free feat at 1st level; count AP as a feat). Used the races from the UA Eberron, but did custom Dragonmarks.