I wonder how much can actually be optimized due the reduced number of instructions found in risc processors. Is is the case if they need some special instructions they can still fit them in an arm processor ?

Everything SIMD that relies on any flavor of SSE or AVX will need to be ported to NEON (or its variable width new thing I forgot the name). Also, many ARM SoCs have asymmetric cores - a couple i7-like ones and a couple Atom-like ones. If I see 8 cores and start 8 threads that need to sync up at the end, my process will be perceived to run at Atom speeds. At the same time, if I wake up my big powerful core to listen to me typing on a terminal, my battery will suffer - unless I have something that's "hard", I should use the small, less hungry cores.

Software will need to take individual core speed in consideration when scheduling threads if it wants to achieve best possible power consumption at a certain performance level. Today, our software assumes all cores are equal. I'm not even sure these asymmetric cores have the same SIMD widths, for instance.