Often, they are built from small diameter trunks, by necessity the sapwood is needed to get the required thickness. That's how I made them as a kid. They shot well, but I didn't know at the time that the usual method was to remove all the sapwood.
I have some staves that I have roughed out that I intend use the sapwood on the back, no more than 1 or 2 rings. Main idea was just a visual accent.
By all accounts that I have heard they behave at least as well as an all heartwood bow..