Having worked in firmware, I have seen the issue with how things are partitioned between software and hardware. The hardware tends to get concrete requirements

Having worked in firmware, I have seen the issue with how things are partitioned between software and hardware. The hardware tends to get concrete requirements and everything else abstract or complex in dumped on the software. So you have to figure out algorithms which have not been thought through yet or deal with edge cases the hardware cannot deal with properly. These all add complexities to the software which we can't quantify early on. I believe if we specced out the software design to a level we are confident with the algorithms and data structures involved, you could get as predictable a schedule as hardware design.