Hello Mr.Pedantic,
I don't have a full grasp of bigger differences between various usages of modals so I'm not bothering myself with nuances

. But I feel that the same ( or virtually the same) form of modals can lead the listener in totaly different course of thinking than what the speaker really wanted to communicate.It seems to me that no other segment of the English language can do that to such an extent and in such a large number of situations. I mean,roughlly speaking, all mistakes in speech will impede proper understanding but inappropriate usage of modals will sometimes make the listener understand the whole meaning wrong.
All the best and thanks