Linux's xattrs appear to only be an array of bytes, while Haiku's extended attributes are typed (string, mime, int, llong, float, double, bool, icon, or raw). This means a compatibility layer is probably needed to handle encoding types into the value of the xattr. Xattr(7) says there is a limit of 64kb for the value. Some haiku programs allegedly use more, but i dont have an example handy. Further, specific file systems may impose additional restrictions. For instance ext4 requires all xattrs to fit in a single file system block (assuming that the man page I'm reading is not out of date).

For compressed files, HFS+ puts the entire contents in an (xattr né resource fork).

macOS resource forks values are "file-like" with position-based "block" read/write, Linux xattrs is a key-value map with atomic get/set operations on the whole value only.