Almost every agency describes itself as full service, and the phrase means the firm will take responsibility for several disciplines. Whether it staffs them is a different question. The useful version of this claim is a firm strong in two or three areas that is honest about subcontracting the rest, and the disappointing version lists everything and is deep in one thing.
Test the claim discipline by discipline
For each area listed, ask who does it, how many people, what else they carry, and whether it is subcontracted. Subcontracting is normal and should be disclosed. The answer usually reveals two or three genuine strengths and several capabilities that are one person or a partner, which is fine if you know it before signing.
The genuine advantage is coordination
One team holding the whole picture means the search work informs the paid media, the creative is consistent, and nobody has to be the coordinator. That is worth real money where several channels run together, and it is the whole case for full service. It is worth nothing if the channels are managed separately inside the agency too.
When a roster beats one firm
Where one discipline is decisive, a specialist will usually beat a generalist's version of it. A small roster of specialists with one internal owner coordinating them gives better depth at the cost of your own time. Companies with an internal marketing lead frequently do better this way.
The honest middle
A lead agency strong where the work is hardest, with named partners for the rest, disclosed and coordinated. That is what most good full service relationships actually are, and firms that describe it that way rather than claiming everything are usually the easier ones to work with.
Questions people ask about full service agencies
Is full service better than specialists?
Better for coordination when several channels run together. Worse for depth in any single discipline. Which matters depends on where your constraint is.
How do I check a full service claim?
Ask, per discipline, who does it and what else they carry. Two or three real strengths and some partners is a normal and acceptable answer.
Should I use one agency or several?
One if nobody internally can coordinate. Several if you have someone who can, because the depth is usually better and the cost lower.