By far the largest problem I've had is that if you move from region to region (I just escaped the USA back to the UK), there's a complication between subscriptions and regional stores.
You can't switch your region while you have an active subscription - which includes any AppleCare or other long-term (1 year, say) "subscriptions".
Applications like Glucose monitors and Diabetic Pump controllers are distinct between regional stores - a UK pump won't work with US software and vice versa, at least for Tandem.
So I had to wait for the subscription to time out. Cancelling it just means you don't pay on the anniversary, it's still active, and calling Apple support wasn't useful - they tried to help but ultimately couldn't effect any change.
Fortunately my last subscription timed out this month, and I can now use my UK pump with the UK software, linking to the UK hospital systems.
So, just beware if you move continent - you might want to make sure you time it well regarding any Apple subscriptions, which didn't occur to me before I moved.