So, that's the freshman-year CS view of the topic, but back here in reality land the "complicated" x86 instruction format has pretty much destroyed all others and none of the supposed advantages of RISC actually exist. Remember that the whole point of RISC is that the CPUs would supposedly run faster. That hasn't happened. There are no RISC CPUs running faster than state-of-the-art x86 CPUs. POWER8 comes closest, but does not exceed.

The whole RISC philosophy was a huge mistake. Yes, x86 instructions do not map well to transistors, and they have to be unpacked into uops to be executed. This is a form of compression. Having a compressed program image turns out to be a massive advantage. RISC proponents thought that x86 was so complicated they could beat Intel with their simple instruction decoders. That almost, but not really, made sense in 1990 but since then has made increasingly less sense, until today where the amount of sense this makes has hit zero. The x86 instruction decoder is a very small part of the floor plan of a modern CPU and every time they rev the microarchitecture it gets smaller. The number of transistors needed to decode the VEX prefix is like a speck of sand on the beach of a 512x512-bit multiplier.