Dredging up some archery lore here:
Yew was the preferred timber for bow making because the heart wood resists compression and the sap wood resists stretching. The sap wood is aligned with the back of the bow (the outer surface, further away from the archer) and the heart wood is aligned with the belly of the bow (inner surface, closer to the archer) to take advantage of this natural spring-like quality.
A big risk to a bow is the bowstring snapping during shooting. This is because the aerodynamic resistance of the string and the weight of the arrow means that the tension in a bow is released over a (short but) measurable time - it is not an instantaneous release. If the string snaps, you have an instantaneous release (this is still a problem for modern bows with fibreglass and corbon fibre limbs - a dry fire is definitely to be avoided).
This suggests to me that yew may well not be brilliant at shock resistance, and the impact of an axe into wood would be just too much of a shock for a yew handle. Quite how this ties in with Toddy's experience is not clear to me - I would be interested to know the weight of the replica axe head, and how large a cross section of yew was used to make the handle, and compare this to the handle size of a similar weight modern axe. It could well be that the replica axe uses a much thicker handle, which may give sufficient shock resistance.
Cheers