Even if this worked as promised, if someone just murdered your child for example, and they got given a 1000 year sentence that was over in an afternoon from your perspective, would you feel that justice had been done?

You can argue that yes, of course it has, they've lived a millennium being punished for their crime, but I feel the vast majority of people would feel short changed by the process.

Whether that in itself is right or wrong is a completely separate discussion. But you'd have to have it if this tech actually existed and was used as they're suggesting.