This has always been interesting to me. It's been awhile since I read about it last, though, and I remember reading some criticisms or concerns that posits (or unums? what's the difference?) would end up being slower for some reason. I don't remember the arguments though, or where I saw them; I think the idea was that there were some edge cases that were common enough in practice that overall it would slow things down. It would be nice to see a balanced discussion of the ideas (pros and cons).