I'm playing a warlock and have chosen the Raven Queen patron (from Unearthed Arcana: Warlock & Wizard). I had the idea of having my sentinel raven assist me on all skill checks, but this seems pretty powerful; thus, I am wondering if there is anything preventing me doing this according to the rules as written.
The description of the Sentinel Raven feature explicitly says "…you gain a spirit that assumes the form and game statistics of a raven," so this raven does have actual stats, and even a +3 bonus to perception. Therefore, I don't see why it couldn't aid party members with skill checks (or anyone, for that matter).
A creature can't Help if it couldn't do it alone
The rules on giving assistance (and thus advantage) to skill checks are found on page 175 of the Player's Handbook, under the heading "Working Together" (bold added):
Although both points are relevant, the one most likely to come up is the first: the raven can only give advantage to another on a check that it could conceivably succeed at alone.
There are certainly some tasks which the raven could assist you with: for example, it could use the Help action in combat and give advantage to the next attack made against a given target. However, many tasks are beyond the abilities of a raven to even attempt. They cannot reasonably be said to persuade another person, for example, as they do not have the intelligence for meaningful discourse. And they cannot possibly successfully grapple a medium or larger creature since their size differs by two categories from them.
A DM will have to make a ruling on some edge cases. For example, a raven may not be able to read tracks or trails, but might be able to help someone track a wounded foe by smelling for fresh blood or flying around you looking for your quarry and thus give you advantage on a Survival check. But in general, keep in mind that you can only be given advantage on a check if the creature helping you could possibly succeed on the check without your assistance. And at the end of the day, there are a lot of things that a bird just can't do.