I ran a 12th level Warlock lately after having run both a Sorceror and Cleric from 1-12.

Warlock is fine.
People over focus on the spell slots. It’s a minor part of the class.
Your real strength is the combination of invocations, pact, and cantrips.

I was running a Celestial Tomelock, so I has healing dice, extra cantrips, and a ton of rituals.
My spell choices were more focused on utility or encounter length spells, so that I could get the most bang for my buck. Almost all my choices were 3-5th level because why not?
Had soul cage as my arcanum to give me a nice long buff after we took down our first foe.

But yeah, I did not feel any less powerful than the others.
It’s just a gestalt class, that like the Sorceror can be messed up with poor choices.

I wish some of the invocations that used a spell slot, instead let you cast those spells once a day without using a spell slot. If so, they would be much more attractive options.
Kinda like Tricksters escape.