You literally have to deal with devils of all "people". Yikes !
Only one subclass of Warlock makes a deal with a Devil. Every other subclass gains their power from other powerful entities.
Also, what makes a "Devil" any worse than one of the evil Cleric deities? Like, is Bhaal, Myrkul, Lolth, Shar, Tiamat, Bane, Asmodeus etc any better than your run of the mill Devil?
When you can have pretty much the same level of power by going Wizard, Sorcerer, Bard, Cleric, or Druid.
Firstly, Sorcerers are literally determined at birth. You choose being a Sorcerer as much as you choose what race you are.
Secondly, not everyone has access to an institute to study being a Wizard or Bard. Nor necessarily has the faith to gain power from a deity like a Cleric or Druid.
Warlocks allow any Tom, Dick and Harry to instantly gain incredible power, for the price of having to obey the terms of the pact they made with whomever gave them the power (The details of such a pact being dependant on what was decided between you and the powerful entity)
Well that, and also frankly Warlocks play very much like one trick ponys.
Technically, one could say the same about any martial class. Where the "One Trick" is "I hit them with my weapon". (The exception being the 2 martial Bard subclasses because for whatever reason, they get to be full Martials AND full Casters)
Warlock can be viewed similarly to the likes of say, EK, AT, Ranger and FE Monk. They have their primary method of engaging in combat (Either Eldritch Blast or weapon usage) but have a limited capacity to utilize spells as well (Whereby they sacrifice per combat uses for spell variety. So they only get a small number of spell slots, but get to choose from a wide variety powerful spells)
