Focus sync problems are indeed quite common in binoculars of any price range. Eliminating or minimizing them is the reason why Swarovski has the springs on the focus shafts, whereby people readily complain that the focus action demands different levels of force depending on direction of movement. Some have even complained about the sound the springs make.
How problematic diopter drift due to focus sync imperfections is depends highly on the individual user and their eye accommodation. When younger, I could take almost any amount, but nowadays at a ripe non-accommodating age, I'm highly sensitive to it. Since a large percentage of demanding binocular users are no longer young, this is an area which manufacturers should take very seriously.
BTW, for testing whether and how much diopter drift there is, mounting the binoculars on a tripod and examining each tube separately with a booster or another binocular is a good method since it takes away the accommodation of the eye from play. You set the focus on the booster to a fixed setting, and focus the left tube of the binocular so that the image through the booster is sharp. Then, through the booster, set diopter so that both tubes are sharp through the booster. Then, focus closer than the target with the focus wheel, and re-focus with the booster behind the left eyepiece only moving focus from close to far until the image is sharp. Now, check that the left tube is also in focus. Repeat the procedure coming from beyond infinity focus to sharp focus. If diopter drifts, you will readily see it through the booster.
Kimmo