Labernum sapwood works just fine in tension. You have to be careful not to use the sapwood if there is any hint of a black line between heart/sap.
Labernum works just like yew, the sapwood is lower density than the heartwood and can give a mildly lower limb weight than an all heartwood limb.
I personally rate labernum at least as highly as the best yew. Here in England it is hard to get yew with good density (not impossible
) but pretty much all the labernum has been high density. It will make the heaviest warbows just like yew and can handle the same cross section. Labernum is very elastic and has great compression tollerance but also just like yew low bend resistance.