Where in-house wins
High spend, where a salary is less than a percentage fee. Deep product knowledge. Someone whose only account is yours.
Where it struggles
One person means no cover and no second opinion. Hiring well is hard when you cannot evaluate the skill. And a single practitioner sees one account’s patterns, not fifty.
Where an agency wins
Lower spend levels, faster start, exposure to problems across many accounts, and cover when someone is away.
The hybrid
Common and sensible: in-house owns strategy and the business numbers, an agency or contractor does execution. What does not work is both making changes in the same account without one clear owner.