The Chain Pact familiar has at-will flight and invisibility, and the warlock can share its senses at infinite range. Plus, if it does get caught, the warlock can poof it and make it reappear next to him. Even if it gets killed, he can just summon another one at little expense.
Picking locks is not part of his arsenal, granted, but that's burglary, not scouting. And you can't just "throw in some rogue" unless you allow the warlock to do likewise.
Stealthy infiltration might be a better term for what gyor is describing.
The monk will have better bonuses, the monk's shadow step and shadow cloak abilities don't cost ki, and eventually empty body becomes available. It's hard for an invisible familiar to compete with and invisible monk who can also cast pass-without-trace for 2 ki points. The only drawback to the monk is he or she still needs to avoid bright light, but is less likely to get noticed and more durable if needed. Likely better perception too, based on WIS as typical for the class plus possible proficiency the familiar doesn't have.
I prefer to avoid absolute statements like, "As a trickster, scoundrel, or social engineer, the warlock can outshine both the bard and rogue if he puts his mind to it. Built correctly, a warlock can out-scout any rogue or ranger. You can even be better at rituals than any other class, including wizards."
I'm not going to commit to disagreeing, because I miss things too and I'm not sure how you would plan to do some of those, but I have some first thoughts on it.
My first thoughts are that the warlock cannot match the skill checks available to rogues and bards, nor can his or her familiar. Bards have half proficiency or proficiency in everything, and lore bards can use inspiration on their own checks. Rogues don't have bonuses to everything but they do have the most base proficiencies, minimum rolls, and the capstone as a fail-safe. Either can supplement skills with spells.
Chain pact warlocks do have massive range on a familiar that can be invisible. Beastmaster rangers don't have the range but do have access to speak with animals and beast sense to make better use of their companions; the companions progress in proficiency, AC, damage, and hit points with the ranger levels; and something like a hawk (proficient in perception with advantage on the rolls for keen) allows for things like aerial overhead view (familiar can do this too but without the perception bonus) and then when he spots the cave or ruins or whatever, the ranger can use primeval awareness to attempt to learn of what monsters might lurk in said whatever from a comfortable range too. Several suitable companions are proficient in stealth and perception for a similar trick to familiars, but with a much sturdier remote scout.
Tome warlocks can potentially get all rituals from all classes. The caveat on that is first they have to find them, and the wizard has the largest list of rituals already that he can add as he levels without looking for more.
I would be more inclined to state that warlocks have strong advantages in the areas in which they focus. It sounds like a very interesting line of discussion on how warlocks do such things, however. I've found them very versatile so wouldn't be surprised if there are some more gems in there I missed.
