Temporary life skills are small individually and ok at low levels, but become pointless in higher encounters, especially if they do not stack.
Viable theory crafting would allow a combination of say: Darkone's Blessing (4-5 hp), False Life (5 hp), and Armor of Agathys (5 hp) (15 extra hp).
At level 2 this would put a 14 con/16 cha warlock at around 20hp with 15 temporary health just about doubling the hp and suddenly becomes a frontline character able to take multiple blows. This could be considered OP in the eyes of some, but consider the growth of that character.
At level 20 this would put a 14 con/18 cha (assuming they increase cha as they level) warlock around 200hp with 34 temporary health. Now this character can survive about 1 extra blow, giving a viable use to stack temporary hp in a pinch or survive focus fire from 2+ enemies for maybe a round or two.
I am not totally versed in table top d&d mechanics, and had to look up how the hp is provided per each level. If the numbers are off, please correct me.
