Found that the Snippets guard against used shortcuts does not block used shortcuts.
Or at lest it did not do so for cmd + shift + H
Which turns out to be In Whisk for :
Edit > Find > Highlight
So I then changed it to ctrl + alt + H
Which is fine,
But on a whim, added a new snippet and was also allowed to add
ctrl + alt + H for that.
I had to chase down what was using it, luckily I got enough of a flash of the sheet that shows and goes very fast to work it out.
I guess I only see this as a problem if you have many, many shortcuts. Not sure how feasible or likely that really is.