I think alot of this is depends on what actually ends up getting implemented in the game, as to whether its effective or not. Sure, in the tabletop game, being able to run around reading everyone's thoughts at will would be pretty silly. In BG3? Who knows? Right now I think it would be pretty worthless, outside of whatever few niche situations are supported by it, that isnt already handled by the illithid tadpole.
The implementation would require extensive work to be done right and would likely be quite useful given the extensive amount of social situations. For instance, in the "Hunt the Devil" side quest where you potentially can kill an important character without full knowledge, the motivations of each party could be sussed out.
As for the illithid tadpole, it is hinted strongly that using the power comes with a cost down the line - like the Bhaalspawn power corrupted in the original game, the new power is likely to corrupt you as well. Otherwise it would be just a bad implementation that diminishes the value of normal social skills. So there are plenty of reasons why the tadpole shouldn't be so readily made part of the equation.
Dark Ones Blessing on the other hand, I just feel is useful all the time, no matter what.
Dark One's Blessing is mostly only useful when the Warlock actually would otherwise be reduced to zero hp or lower. As a means of healing, it's usefulness is relatively significantly weakened by the very generous amount of healing that exists in the game at present.
As temporary hit points, ut should not stack with any other source of such, like Warlocks second signature spell Armor of Agathys (powerful spell that opens up for some specific builds to maximize the retaliatory damaging effect by means of damage resistance). Dark One's Blessing, which is already only conditionally useful for reasons given above, is with Armor of Agathys, potentially a negative asset that removes the aforementioned spell.
I just like the idea of the Great Old One better than the Fiend.
I got that impression pretty loud and clear lol. The Fiend can be roleplayed to mean something other than selling your soul to evil/the devil though. The fiend could be roleplayed as a fallen angel on a path of redemption. Or you could have made the pact with a devil to save your loved ones. Anyway, another reason as to why I want the HEXBLADE subclass implemented; the subclass is already tied into Baldur's Gate by proxy of Blackrazor which was a powerful sword in BG2 and offers another roleplaying option.
