British beekeepers have an interesting little aide-memoire:
A swarm in May is worth a stack of hay.
A swarm in June is worth a silver spoon.
A swarm in July isn’t worth a fly.
In other words, swarms in July or later will likely not survive the winter, and won’t make you a good honey harvest that season. As a split is a kind of artificial swarm, the same wisdom could be applied to doing splits. 
Just my way of looking at it, but many British beekeepers think that way too.