Why mutations should be synchronous?

You need to remember that mutation handler functions must be synchronous. This is why because any state mutation performed in the callback is essentially un-trackable. It is going to be problematic when the devtool will need to capture a "before" and "after" snapshots of the state during the mutations.

mutations: {
someMutation (state) {
api.callAsyncMethod(() => {
state.count++
})
}
}