I would expect epoxy glued and coated sinew to be water proof and probably superior to hide glue adhered sinew. The same for sinew glued with Titebond III and coated with polyurethane. But, epoxy, Titebond III or Polyurethane are not primitive materials. They are not illegal, immoral or fatening, so it depends on how primitive you want to be. You will also get superior performance from fiberglass or a compound bow, so each of us needs to draw our own chosen red lines.