A minority of drivers/GPUs are blacklisted on desktop, and you have to deal with less than 100% WebGL coverage in any case. But yeah on mobile it's a sharper "reasonably recent devices only" division.
Be careful to interpret the stats there correctly. Eg when you leave just the specific category checkmarked, the graph shows ios-smartphone webgl support rate at 79% and android-smartphone rate at 75%. The percentages in the sidebar are just browser shares and not WebGL support rates.
You can see (somewhat skewed) stats on http://webglstats.com/
Interestingly iOS is ahead of Android in tablets, but behind on phones.