From my experience agencies around 200 employees that specialize seem to be the best. “Full service” means shitty at everything. Too small of an agency and you don’t get experts (or the experts leave), too big and you’re stuck in bureaucracy, huge margins, occasion very under skilled employees who go unnoticed, etc.

I’m also starting to see more often the game brands play if switching vendors constantly so that each vendor operates at a loss to try and win the clients business. But the client always leaves by design.

The truth is that with performance marketing the know-how stays in-house. Agencies can't compete with r&d and dedicated analytical and engineering resources big brands have. The services offered are "for the rest of us".