Simplicity of what exactly? From the perspective of the device/hardware, SPI is much simpler. A UART based solution tends to be relatively much more complex on the hardware/device side. You can implement a basic compliant SPI slave in a tiny bit of silicon - like a handful of flip flops simple. Moreover, you realize many of the simplest SPI devices don't have an oscillator, so you're still going to need a clock line - and if its truly a UART/asynchronous it may not be convenient to get a high enough speed external clock.