The primary function of a boot is protection. There is little evidence to suggest that boots can provide support, although boots are commonly sought out for this use too. The problem is, you can’t tell how protective a boot is, just by simply looking at it.
The only way to tell if a boot is protective enough is for it to be tested under controlled conditions, to forces that we can replicate (such as when a horse hits a fence, over reaches or strikes itself with another limb). There are some other factors to be considered too, which can potentially hinder performance or increase the risk of injury.