The problem with a finger sling or wrist sling on a longbow is...properly adjusted( ie loose enough) is that there isn;t enough shelf protruding ona longbow for the sling to keep the bow from sliding through the sling and hitting the ground..although I guess thats better than flying through the air and then hitting the ground.Downside is, without complete confidance in the sling retaining the bow....ya end up grabbing at the bow at release...and slo-mo video made believers out of several who swore they didn't grab the bow.
I think Bateman sells a simple lace on bow mounted sling , while the bow mounted slings are not as good IMO as a finger or wrist sling, they do instill confidance in retaining a longbow and may keep you from devoloping a habit of gragging at the bow.