Lots of people are unsatisfied with the warlock. Eldritch Blast works perfectly. The problem? Eldritch Blast is the only saving grace of the class. That's why its constantly spammed. And its supposed to be optional. And everything else (from lack of fiend summon spells for the Fiend'lock, to crappy Invocations, to spell slot/short rest issues, to crappy chain and blade pact options, to AC issues without feats, to Hex's Concentration issues, to difficulty with viable new subclasses, etc.) is highly controversial. The warlock also theoretically had a similar vibe to the Pathfinder Witch class and its curses, but falls flat in that as well due to various issues. And, laughably, you can't even use Eldritch Blast on the Tarrasque or Tiamat, if your game includes either.
While I won't say that the class is broken, it does have a lot of problems that really should be fixed.