Selling 35 million units in four years is hardly a failure, imo. With sales trending upward this year, amd witha good three years left, the X1 is likely to surpass the 50 million mark too. Which would be ahead of the SNES.
So does that mean SNES is a failure also?
You obviously can't compare the SNES generation with this generation. The gaming industry was tiny back then. Xbox One is a failure compared to its predecessor and compared to its competitors and those are the only relevant metrics.