Why is the deposition of silver more uniform from [Ag(Cl)₂]⁻ (aq) than from Ag⁺?
The uniformity of a deposition depends on more parameters, but in this case the difference is in electromigration effect. $\ce{Ag+}$ ions are atrracted to the electrode surface, but the complex $\ce{[Ag(Cl)2]-}$ is repulsed. Cyanides form  complexes as well: $\ce{[Ag(CN)2]-}$ or $\ce{[Ag(Cl)3]2-}$ and they are also  repulsed. 

Silver electrodepostion form $\ce{AgNO3}$ ($\ce{Ag+}$ in solution) is without additives nearly impossible as dendrites are created. Fukunaka, Yamamoto, Kondo, Ionic Mass Transfer Associated with Dendrite Growth of Electrodeposited Silver in AgNO3 Solution, 1989 ECS.

The metal deposition from a complex molecules is described in Wilkinson, P. Understanding Gold Plating. Gold Bull 1986, 19, 75 – 81



Some chemicals, that are usually added to electrolyte, affect the deposition because they create a film as they are adsorbed at  inner Helmholz plane. (E.g. acidic Cu deposition with alcohol addition Popescu et al, Materials Chemistry and Physics 72 (2001) 332–336.)