In the past few years there have been some large storms that missed Tokyo by a fairly small distance. The people who operate this system say that it would have been over capacity if those storms had hit Tokyo directly.

It's an impressive system but there are definitely still storms large enough to overwhelm it. But, combined with the other flood defenses in the area, it has made flooding less common in all normal weather scenarios, which is a big win compared to the situation before it was built.