One problem I see is when they write the pairs of metadata blocks for redundancy, in many FTL implementations they might fall on the same physical page of flash

One problem I see is when they write the pairs of metadata blocks for redundancy, in many FTL implementations they might fall on the same physical page of flash so there is not much redundancy. When FTL write metadata for their own needs, they are congnizant of the flash layout and make sure to write metadata to separate die/block/page.