I conducted a quick Twitter poll which I closed at the start of this month that explored the key challenges the average digital marketer faces, there were quite a few but here were the most popular:
Interestingly, many of the principles I covered in the 4HSWW can help overcome all of these challenges. A modern SEO or even a team of modern SEOs can't necessarily specialize in all aspects of the practice any more, the knowledge requirements and rapid pace of the industry are mind-blowing. I'm not saying it isn't possible but it might not be preferable. There are many situations where it makes sense to either explore bringing in a 3rd party supplier that specializes in that very specific thing or a contractor who can support the skills you do have in house.
In the modern landscape there isn't anything wrong with a team of very capable generalists (who perhaps have one specialism each) overseeing a network of suppliers and contractors who specialize in certain areas - Moz labelled this "The T-Shaped Marketer". That setup is likely to yield the best results for both in-house and agency. There is often a stigma attached to "outsourcing" especially in the big agency world but the fact is there is still value being added by pulling together a network of world-class suppliers and intelligently directing and overseeing them. I accept you're probably not earning your keep by just farming out an entire contract, however, supplementing internal capacity and augmenting internal capabilities is a very intelligent decision.
This isn't just about people either, a 3rd party provider (be that supplier or individual) may have a very different process to yours. If your agency handles a diverse range of digital projects, it is entirely possible you have a boilerplate workflow that you then have to shoehorn every new service into. Whereas a 3rd party provider may well be 100% focused on that specific thing not only attracting the finest talent but constantly refining the process.