Is there a reason not to take the optional class features from Tasha’s Cauldron of Everything?

Tasha’s Cauldron of Everything includes optional class features for every class from the Player’s Handbook. Barbarians, for example, can choose to gain an extra skill proficiency at 3rd level, and/or choose to gain a small bonus move as part of the action they take to start raging at 7th.

These features are listed as optional, but they don’t seem to have any drawbacks attached, besides a note that you should consult with your DM about taking them. Given DM approval, is there any good mechanical reason not to take an optional class feature, or should they essentially be considered upgrades over the base class?