Bit fields do suffer from a lack of portability between platforms. Why?

Bit fields do suffer from a lack of portability between platforms. Why?

Bit fields are a convenient way to express many difficult operations. However, bit fields do suffer from a lack of portability between platforms:

• Integers may be signed or unsigned
• Many compilers limit the maximum number of bits in the bit field to the size of an integer which may be either 16-bit or 32-bit varieties.
• Some bit field members are stored left to right others are stored right to left in memory.
• If bit fields too large, next bit field may be stored consecutively in memory (overlapping the boundary between memory locations) or in the next word of memory.