Waves, rivers, and tides shape delta morphology. Recent studies have enabled predictions of their relative influence on deltas globally, but methods and associated uncertainties have remained poorly known. We address that gap and expand on earlier work that quantified delta morphology within the Galloway ternary diagram of river, wave, and tidal sediment fluxes. We assess delta morphology predictions compared to observations for 31 deltas globally and find a median error of 4% (standard deviation of 11%) in the river, tide, or wave-driven sediment fluxes. Relative uncertainties are greatest for mixed-process deltas (e.g., Sinu, error of 49%) and tend to decrease for end-member morphologies where either wave, tide, or river sediment fluxes dominate (e.g., Fly, error of 0.2%). Prediction uncertainties for delta morphologic metrics are more considerable: the delta shoreline protrusion angles set by wave influence have a median error of 45%, the delta channel widening from tides 25%, and the number of distributary channels 86%. Larger sources of prediction uncertainty are (1) delta morphology data, e.g., delta slopes that modulate tidal fluxes, (2) data on river sediment flux distribution between individual delta river outlets, and (3) theoretical basis behind fluvial and tidal dominance. Future works should address these sources, improving predictions of delta morphology from sediment fluxes that allow us to assess how much climate and global changes affect deltas.