They get a massive power bump at 11th level. They suddenly get 3 slots per short rest and a daily 6th.
If you compare a warlock to another caster, they get (for the most part) just as many 6th, 7th, 8th and 9th level spells (excluding the 6th and 7th slot at 19-20), and it's only 5th level slots and below where there is a difference. A warlock get's fewer slots but their slots are far more top-heavy.
Compare an 11th level warlock to an 11th wizard.
Warlock: 3 x 5th level slots (6 with 1 short rest, 9 with the recommended two)
Wizard: 4 3 3 3 2 for a total of 15 slots.
Even with just one short rest in a day, I'd argue that 6 x 5th level slots is comparable to, if not preferable to 2 x 5th and a bunch of lower level slots, especially when you take into account the warlocks invocations which tend to take up the utility role that low-level slots are used for. With 2 short rests a day, I'd argue the 9 x 5th level slots is far more preferable than the wizard slots.
When a warlock reaches 17, things get even more interesting:
Warlock: 4 x 5th level slots, 8 with one short rest, 12 with two short rests.
Wizard: 4 3 3 3 2 for 15 slots (same as before)
Now we can definitely see that a warlock with 2 short rests gets 12 x 5th level slots a day compare to a wizards 2 x 5th level slots and associated lower level spells.
I'm not arguing that a warlock is a better caster than a wizard, I'm saying they each have their benefits. A wizard is far more flexible in his options and can burn through all their slots in a single combat, over an extended adventuring day though, a warlock has far more staying power as a caster.
Take into account a warlocks strong cantrips and and access to other at-will utilities, and potent class abilities and I think they make fine pure-casters.
I'm currently playing a Warlock (Fiend, Pact of the Blade) 12/Fighter (Battlemaster) 3 and find I can function perfect well in the caster role despite being multi-classed.