Yes I have sinew backed a few bows. I didn't find it all that difficult... Time consuming, messy, and slightly tedious, TOTALY.
The boo failures is one of the reasons I was a little hesitant in going that route. Hickory does sound like the best option in terms of ease of use.
I guess I should have clarified a little better in that I am not after the easiest, or simplest method, I just had not seen anything mentioned about flax in my research about ipe backings. I was simply thinking it might make be a durable alternative and appropriate backing, but as pat b explained there are some problems with it.
Thanks for that insight DC, it's that sort of info I was looking for.