>("Add buy ketchup to my shopping reminders" failed, but "Add ketchup to my shopping list" worked)
This is exactly the way I would expect it to work. Would you say to a personal assistant: "Add buy ketchup to my shopping reminders"? It isn't natural. The second phrase is much more natural and was understood as one would expect.
If I wanted a list of work reminders, I might have: "Add send contract to john to my work list." which would be less natural... also, in my mind, what I was adding to my reminders was the task to buy ketchup, not "ketchup" by itself.
Anyway, I'm sure they'll add more supported syntaxes, and in parallel people will learn and adapt to the syntaxes that are actually supported.