When they say "The effect of mandatory checklist implementation is unclear" they really mean that the magnitude of the effect is still unclear.

Also, the metaanalysis describes which studies have gotten the same profound results:

"Only studies including team training or a more comprehensive safety system that includes multiple checklists have shown effectiveness similar to that seen in the WHO study."

Maybe the benefits are overhyped, but there appear to be reductions in mortality during hospital stays, and the studies suggest ways hospitals can improve results (ie, team training and systems with multiple checklists).

Of course, there was not enough data yet to conclude any improvement within the 95% confidence interval, but still, it does hint at a great reduction in serious mistakes.