Why is synchronizing every bit of an FPGA data bus separately unsafe?

Different bits can be captured on different destination cycles. During a multi-bit transition, the destination can combine old and new bits. Independent single-bit synchronization addresses each bit's metastability risk but does not preserve a coherent word.

For infrequent updates, use a validated handshake scheme that holds the data stable while control crosses and capture completes. For a stream, use a properly implemented asynchronous FIFO. Gray coding is useful for specific transitions such as FIFO pointers, not a universal encoding for arbitrary changing payloads.

Review the chosen structure's reset sequence, throughput and timing constraints. Exercise rollover, bursts and near-full conditions for a FIFO. Prefer supported CDC macros or verified IP to an improvised crossing whose assumptions are not captured by either tests or constraints.