As far as I know RISC provides similar power efficiency and sleep that is like ARM.
ARM is Western
RISC is China / Eastern
Valve is just trying to outflank Microsoft here. And they're doing a magnificent job of it.
Microsoft has on at least half a dozen occasions tried to draw a box around Valve to control their attempts to grow beyond the platform. And moreover to keep gaming gravitas on Windows. Windows Store, ActiveX, Xbox, major acquisitions ... they've failed to stop Valve's moves almost every time.
Linux, Steam Box, Steam Machine - there's now incredible momentum with a huge community with more stickiness than almost any other platform. Microsoft is losing the war.
The ARM vs RISC battle will happen, but we're not there yet. There also isn't enough proliferation for it to be strategic to Valve.