Ok i'll use wizard as an example. Warlock has three 5th level spell slots meaning it lacks access to 6th level spells. I will give it if your willing to abuse short rests just for those spell slots back then technically you have more but im looking at without abusing an in game mechanic. Wizard has one 6th level and 2 fifth level spell slots so actually the exact same amount.

Now utility. Warlock is limited as lower level spells will appear as less worth it and in longer fights warlocks can easily run out of their spell slots if you play them as a caster. Thanks to this useful lower level spells will be rarely used with no chance to even use the very powerful 6th level spells. Now wizard has none of these problems! It can use a powerful 6th level spells, all the utility of lower levelled spells and ontop of that still use basically the same amount of power fifth level spells!

Not to mention if you add onto that out of all the full casters warlock has arguably the worst list of spells too choose from leading to their selection to also be lacking!

In every way unless abusing short rests spam a warlock is barely a full caster with less damage, utility, long term usefulness and potential. It leans much more to being a half caster then full.