I confess that I find myself clueless with the 8th level spells available for the Mystic Arcan Warlock function, especially if you for example. to have one or more bards in the party (as I do).
(Maddening Darkness seems to have such a huge radius that it would disable your party, Glibness and Dominate Monster seem covered with bards, Power Word Stun and Demiplane look disappointing, and I'm not sure what I think about Feeblemind – I saw an assertion that pouring was probably less ethical than making your deal in the first place.)
Honestly, the 7th level spells all look much more interesting, flexible and useful. (Etherealness, Forcecage, Star Crown, Airplane Change and Death Finger all look like much more flexible choices or better to fill niches that bards can not cover, especially the role of blaster.)
I see in this question that it is not consistent with RAW to choose a level 7 spell (instead of a level 8) for their mystic arcane at warlock level 15, but imbalance?