Sure, strictly speaking it's impossible to ensure that a message was actually read by a user even with automated end-to-end delivery acknowledgements and/or in centralized systems: UIs manage to gobble/hide messages, users fail to find how to open attached documents (and declare that those are missing), etc. But I imagine that a survey/statistics would still help to estimate how bad deliverability in general (in a variety of common cases) is: without that there are differing and even more vague ideas of its state.