I have done hundreds of ZF overhauls with no reverse and have never seen the reverse drum shred. There is a snap ring that in my opinion is too close to the edge of the drum considering the amount of pressure that is used for reverse, The edge of the drum cracks off just above the snap ring allowing the piston to push the clutches out too far which lets the piston in the reverse drum get crooked in the bore and shears off the seal which reduces the fluid pressure. The car might still have reverse when the fluid is cold because it is thick and can still apply enough pressure to the cluthes cold but not when it warms up.
Once the piston seal is torn there is reallly no way for the drum to shred because it doesn create enough pressure to work. The forward gears will still work because that drum and clutches is only used for reverse. I see a lot of people spend lots of money gettng this fixed when they don't need to. The drum with seal kit is like $400 and they are ZF parts. I vry rarely repace the clutches even with 100k thay are usually You can get them from wittransmissions.com
The hard part is finding someone to fix the trans with customer supplied parts. The actual time for removing and overhauling is like 6.5 hours