It's relatively balanced
I've DM'd a highly min-maxed Raven Queen warlock from level 1 to 14 and I've never felt that it unbalanced my game.
That said, this subclass excels in certain scenarios and is quite weak in others, which I've described below.
It's strong out-of-combat
The bonus to Wisdom (Perception) checks and the effectively permanent access to a familiar provides a high degree of scouting capabilities. Additionally, the fact that the warlock doesn't need to spend gold on summoning the Raven (as compared to Find Familiar) affords them considerable flexibility in using the Raven as a scout.
At level 6, Soul of the Raven provides out-of-combat flying capabilities which can trivialize certain encounters. That said, at this point in-combat flight is easily attainable via the "Fly" spell (among a multitude of teleportation options) so it's a negligible issue at worst, especially considering that the action economy of Soul of the Raven makes it mostly useless in combat (as it requires a full action to dismiss).
All of this is offset by the following:
It's relatively weak in combat
Most warlock subclasses provide in-combat features that either increase durability or increase damage. In early levels, all of this is absent in the Raven Queen warlock. As such, it is considerably weaker in combat scenarios compared to combat-oriented Patrons like the Archfey and the Fiend. At early levels this is likely to be very noticeable.
This is offset by a somewhat combat-optimized spell selection. Spiritual Weapon, in particular, synergizes extremely well with a singly-classed RQ warlock due to the strong scaling of the spell and the lack of other bonus action options. Cone of Cold, of course, is strong as well.
At later levels, the combat ability of this class increases considerably, but this holds true for other warlock subclasses as well.
The only issue I've faced with this class is the clunky wording on Sentinel Raven and Soul of the Raven. Specifically the verbiage on the former allows an RQ warlock to cast "[x] you can see" spells on things they themselves cannot see (as they can see through the Ravens eyes at no cost), while the former has unclear interactions with spellcasting.
That said, these are exactly the sort of oversights which are typically corrected in an official release, so, assuming the DM takes a RAI interpretation of the rules, these issues should be easily avoided.
This subclass has clearly defined weaknesses and strengths. I've never had issues with it and personally it's been one of my favorite subclasses to run as a DM. It's probably closest to the Great Old One warlock; extremely flavorful, with strong out of combat utility and a back-loaded in-combat power progression.