Darell Havener and I have had an ongoing discussion about growing roots for ficus both under ground and aerial. That is one advantage of being retired, you have the time to ponder the really important questions.
Anyway about 5 years ago we were considering whether Coir (basically shredded coconut husks) would be a good medium. So in January of 2015 I planted a willow leaf ficus in coarse coir (1/4 to 1/2 inch cubes) in a colander. The thought was the coarse coir would allow plenty of oxygen and retain moisture. In the original planting the coir came above the two trunk aerial roots you see. They developed between 2015 and 2017. Since 2015 then I have pruned it a couple of times. It dropped its leaves once or twice.
Today I pulled it and repotted it. The experiment was a failure. See the photo below of the roots. That is pretty much how they looked in 2015 when I started and in 2017. The fine roots had grown out to the colander, but they break off easily when eliminating the coir. The trunk got fatter in the five years.
For this repotting I wired the two aerial roots coming off the trunk to the trunk in hopes they will fuse. That is a Bruce Murdock pot I bought at the Christmas auction last year.
Coir may be good for orchids but I get better results with other substrate mixes.