Crappy spells. You get fireball at 9th level, as opposed to 5th when the damage is significant. All hybrid casters suffer from using full caster's hand me downs many levels after they were relevant. You're still blasting every round because otherwise you're the chump casting a first level burning hands for 9 damage.
If there were more hybrid spells that were GOOD at the level hybrids got them, it would be different. This warlock is 2 level dip material to pick up fighter DPS, then bounce back to full caster.