They could use it in a group of minions and stack it up and get a big heal at the end of it.
Not really, because wolf's attacks are pretty slow, so stacking it in a group of minions would take some time. Junglers don't really can afford such waste.
Kindred has an ult that grants them survivability so they can stack that heal while they should be dying, have more HP than you, and then kill you straight out.
It would depend on ult's and W's timing. Sometimes, Wolf's Frenzy can end before ending of Lamb's Respite which means - no heal at all (heals doesn't work in the circle).
In other case, when both duelists are low, W's dmg won't stack because of obvious invulnerability.
And finally, when Kindred are low on hp, but there is Kha or other Rengo that wants their head. Well, that is potentially problematic because at the end of Lamb's Respite, Kindred would be healed for 200-300hp (but enemy would be too, so it doesn't really matter) and some additional health points from W. However, it's a perfect scenario where W ends as soon as the R does, or where it was placed in a way that lets you jump out of it right after the blessing ends which requires some prediction. Thus, it would be a new mechanic, that is not so easy to execute properly, allowing enemies to surprisingly catch The Eternal Hunters with low hp and kill them as soon as their R ends because the heal wouldn't actually proc fast enough. Unless they would flash or something.
Hope you get my point.