One annoying part about floats is the abuse of the NaN space. Some runtimes like SpiderMonkey, JSC, and LuaJIT abuse the NaN space to store pointers in doubles. This practice is often called nan-boxing, but has a few variants. A more efficient use of bits like with Posits will break this.

As for the claim about a FPU taking up less space and power for posits than floats, facebook made that claim: https://code.fb.com/ai-research/floating-point-math/