That's only the case in the first write. Rewritten blocks suffer from severe write amplification in SMR drives due to much larger segments needing to be written even for small changes.

Rather than ideal, a rebuild is actually the worst case scenario as the RAID may detect the drive as unresponsive as it scrambles to catch up and thus gets dropped, exacerbating the problem for the other drives in the array. This is HL2 Resonance Cascade bad.