The ISA isn’t the biggest problem. The memory model (do I need to use memory fences here? How many different kinds of those are there?) can introduce hard to reproduce bugs.
Also, relative strengths of the OS (if thread creation is relatively slow, it may not be worthwhile to multi-thread a short code fragment; one OS might have three kinds of spin locks, another only two; if there are many registers, you may need larger stacks per thread, and may want to limit the number of threads in memory-constrained devices) are bigger ones.
Also, relative strengths of the OS (if thread creation is relatively slow, it may not be worthwhile to multi-thread a short code fragment; one OS might have three kinds of spin locks, another only two; if there are many registers, you may need larger stacks per thread, and may want to limit the number of threads in memory-constrained devices) are bigger ones.