That's not wrong, but it doesn't help that the blade and chain warlocks are sub-par options with little feat or in-class support for what they want to be.
Which is an absolute disaster from the game design standpoint. Simplicity and ease of play is supposed to be a selling point. A low level of system mastery to play effectively. With the warlock, its exceptionally easy to make bad choices and have a character that doesn't do much.
Because of several reasons. 1) If we assume no multi-classing or feats, we're looking at pretty much restricting the warlock to DEX-based weapons, thanks to the restriction to light armor and no shields. 2) Without feats, that eliminates the almost-required Warcaster, which means we MUST restrict our offhand to manipulating material components or somatic gestures. This means the blade warlock is, without doubt, a DEX-duelist by necessity, no other options. That means we're restricted to a rapier, short sword, whip, or dagger. You could, in theory, set up your limited spells before combat, and go TWFing, but that means neglecting Hex (both are bonus actions to use), and Hex with Extra Attacks is better than a single off-hand attack.
Given that throwing the dagger is actively dumb (why would you eliminate your pact weapon when you need it for Extra Attacks), and short sword is just flat out inferior without a TWF option, the rapier and whip are the only two choices here. With Hex up, we're doing equal damage to a Fighter with a greatsword. The extra THP puts us on par with the Fighters better HP per level and second wind.
So far, so good. Now, here's where things go sideways. Hex is a Concentration spell, and a hexblade is a melee warrior. He's going to be taking hits, and trying to make multiple CON saves without Advantage or Proficency. Without feats or multiclassing, there's no way for him to make the saves with any regularity. That means we lose Hex. We don't have enough spell slots to keep it up constantly. Suddenly, our damage output stalls / dies. We actively have a character that's doing less damage than a Fighter. We don't do enough damage to meaningfully affect combat. Our chances of activating Dark One's Blessing drops. We could use TWF at this point, but its a bandage still leaves us with subpar damage compared to anyone else, with less AC.
Meanwhile, the Fighter has Superiority Dice, spells, or whatever the Champion gets, and Action Surge, and consistent damage output. The paladin, without feats, can generally out damage the Fighter. Rogues stay consistant with damage output and have their nifty reaction to halve damage, plus the ability to run around the field with Cunning Action. Valor Bards get plenty of options as full casters plus their Inspirations, as do the War Clerics with their buffs and heals. Rangers can do the Hunter Mark thing as a duelist, but get a shield and +2 damage from Dueling Fighting Style.
For the vast majority of players, its not fun to be consistently out shined and have limited build options. THAT is the problem. Without system mastery (which requires multi-classing and feats), the blade warlock is a subpar option that doesn't deliver on its promise of being a spellblade style battler. Especially when they could be doing better damage using Eldritch Blast from ranged while getting hit less.
