What are the key disadvantages and limitations of SPI?

SPI's simplicity comes with real limitations that can make it the wrong choice for certain designs:

Pin count scales with slave count. Each additional slave requires a dedicated CS line from the master. With 8 slaves, you need 11 GPIO pins (SCK, MOSI, MISO, plus 8 CS lines). If GPIO is scarce, this becomes untenable. Workarounds include using a decoder IC (3-to-8 decoder turns 3 GPIOs into 8 CS lines) or daisy-chaining, but both add complexity.

No built-in error detection. Unlike I2C (ACK/NACK) and CAN (CRC + 5 error mechanisms), SPI has zero error-detection capability at the protocol level. Every byte is accepted as valid. Application-layer checksums or read-back verification must be added manually.

Single master only. The SPI specification does not define multi-master arbitration. If two masters drive SCK simultaneously, the bus is corrupted. In systems that genuinely need multiple bus masters, I2C or CAN is a better choice.

No formal standard. Unlike I2C (NXP specification) and CAN (ISO 11898), SPI has no official standard document. Different vendors implement variations: some devices are MSB-first, others LSB-first; word sizes range from 8 to 32 bits; some devices use CPOL/CPHA Mode 0, others Mode 3. Each device's datasheet is the only authoritative reference, and you must verify compatibility for every new part.

Short distance only. SPI uses single-ended signaling referenced to a shared ground. Over cables longer than 10-20 cm, signal integrity degrades rapidly due to ground bounce, crosstalk, and capacitive loading. For board-to-board communication over cables, consider LVDS-SPI or switch to a differential protocol like CAN or RS-485.