In one of the threads (mattes one, I think) someone (itimpi, I think) describes one scenario where the problem occurs.
Its simple - no one is saying you can't use GD, all that the authors of calibre, and support volunteers, are saying is "GD is has been known to cause problems with calibre", so if you use it and you run into problems you won't get a very sympathetic ear.
If you read the calibre FAQ you'll see a number of 'warnings' about products that have been known to create problems - SpiderOak, Watcom, Roboform, Spybot etc, etc. No doubt there are people who are using them without any -ve impact, but ...
The GD problem is the major reason why Charles Haley the developer of Calibre Companion (and significant chunks of calibre itself) has this in his FAQ, see ==>>
Why does CC warn against using Google Drive?.
At one stage CC didn't support GD at all, although it does now - but with a caveat emptor warning.
BR