Is is_writable() sufficient in WritableFileSystemValidator?

Problem/Motivation

Right now, WritableFileSystemValidator uses is_writable() to check if the paths are writable. Is that sufficient for our purposes? Can it return false negatives or false positives due to, say, exotic filesystems or SELinux? Even if it can, are those things likely enough for us to handle them in a special way?

Proposed resolution

TBD, if any.