Last I checked (and things may have changed since) is that svelte didn't support transparent reactivity for deeply nested objects. So something like `$myStore.foo.bar = "baz"` wouldn't trigger a component update, you need to remember to use .set, .update etc. You could assign to local reactive variables but you couldn't assign to stores.
I am sure the library authors made these trade offs thoughtfully, I was just disagreeing with the blanket statement that Svelte is generally a major improvement over Vuejs.