I'm going to go out on a limb and say that heat tempering will improve the compression characteristics of any wood. However, a wood that tends to be brittle and prone to chrysalling (eg hickory) is still going to be prone to chrysalling when heat treated. The key is figuring out what you can get away with as far as limb thickness and degree of bend. If you've had a failure, you have a data point. Next time make the limb a little wider/thinner, or a little longer, or tiller it a little smoother, and heat treat it also, and you have a higher probability of success.