I'm allowing Tasha's in (only) my Odyssey of the Dragonlords game. There's a Twilight Cleric.
My overall feeling is that Tasha's breaks the game balance of the 2014 rules, but does not break the game, IF you are running an epic or mythic fantasy campaign. I would hate it in my more swords & sorcery or low power themed campaigns. There are a few fairly broken things in the 2014 rules, but they have to be searched out, whereas Tasha's seeks to give you them on a plate. In Odyssey of the Dragonlords when the PCs trivialise an encounter, it feels appropriate, like an episode of Hercules: The Legendary Journeys with Kevan Sorbo laughing at the mooks. I wouldn't want that feel in every campaign.
My overall feeling is that Tasha's breaks the game balance of the 2014 rules, but does not break the game, IF you are running an epic or mythic fantasy campaign. I would hate it in my more swords & sorcery or low power themed campaigns. There are a few fairly broken things in the 2014 rules, but they have to be searched out, whereas Tasha's seeks to give you them on a plate. In Odyssey of the Dragonlords when the PCs trivialise an encounter, it feels appropriate, like an episode of Hercules: The Legendary Journeys with Kevan Sorbo laughing at the mooks. I wouldn't want that feel in every campaign.