From a GTD standpoint it doesn't matter. Cloud-only, on-premise only, something in between... from a GTD perspective it's all good.
Avoiding the cloud is like building a conventional brick wall to stop air traffic. If you pay taxes, have a driver license, use a bank, go to the doctor, hell, if you even exist your data is stored in places you know nothing about and have no control over.
I use a cloud-based list manager for GTD. I store documents and spreadsheets in OneDrive. Those have never been hacked. Any time my data has ever been hacked it's been from places like my bank, some level of government or "Who the Hell Are They, Inc."
I suppose you could insist on getting all of your bank statements and bills on paper. You could disconnect from any computer networks (of course, that would mean no internet, which would mean you're not reading this). You could go full-on survivalist and go build a log cabin and an outhouse in the woods and live there (in the log cabin, not the outhouse, don't be disgusting). Even then there is still personally identifiable information about you floating around and you have no control over it!
That's why I opt for having access to the internet and indoor plumbing.
Just use your GTD system to cope with life's curveballs. You only really have control over the choices you make.