What evidence do you have against Pope John XXIII that would thwart his canonization?
I have no evidence to thwart canonization. We don't have a "devil's advocate" any more so as far as I know no one has evidence to thwart canonization.
But the burden of proof is on those advocating canonization. Ordinarily, two verified miracles due to the candidate's intervention are required for canonization. Pope Francis has waived this requirement for John XXIII.
John Paul II however does have two verified miracles.