It's a good question. Sounds like we should document or write up a blog post about the gory details.
Recall that writes always create new objects. The storage nodes are selected dynamically for each write, so in practice that won't fail for storage node failure.
The more likely way to get a 500 from the data path is if parts of the metadata tier become unavailable. As with storage nodes, such failures are typically transient. They're also unlikely to affect reads. Writes may experience transient errors as the metadata tier recovers from failures.
[Edit for grammar.]