Synthetic personas catch the obvious winners and obvious losers reliably. That's where 80% of ad-testing decisions actually sit. They're less reliable on ambiguous middle-ground creative where real human nuance matters.
Use Ad Lab to kill the clear losers before media spend and ship the clear winners with conviction. Don't use it to make final go/no-go calls on campaigns where you're betting hundreds of thousands.
Also: pure visual creative (no text) isn't supported yet. Ad Lab tests anything with words — copy, headlines, landing page hooks, email subject lines, positioning statements. For visual-only creative, the tools don't exist yet that do this well.