Sinew by itself works OK. Glue by itself works better than sinew. But the combination of glue and sinew works GREAT. The style of the arrowhead also determines how it will be attached.
One purpose of the arrowhead to is provide a way to put a hole into the target and cause bleeding and/or puncture trauma. It only needs to work once, in this case. Eventually, the archer gets to know what the minimum amount of wrapping/glue will allow the arrowhead to do its business.
Also, there are descriptions in the old literature from the frontier that some arrowheads were tied on so that they would become loose in the wound and remain inside the wound or make it very painful to remove. Some arrowheads have only one notch on one side, for example. It was said that this made the arrowhead "turn" inside the wound if the arrow was pulled and caused "extra" misery to one's enemy. So, there are sometimes reasons for an arrowhead to be "imperfectly" attached.
Clear as mud?