Ultimately, we need to consider ROI that the campaign should be produced. You will need time investment for your SEO services through existing employees. At that time, good expenditure on the company’s current, regular work may be incurred. But assume that a contractor has been brought for this purpose. Compared to the amount required to outsource campaigns for a reputable agency, it will actually cost more in the wages.
These factors take me to the conclusion that the most effective processing agency for SEO is done by a company. It is the most cost-effective solution and the search engine takes at least risk on the website in case of potential penalty.