Probably my main fear is based upon having several cards each with either subtly or starkly different T&C on insurance. Traversing which card needs to be used to book airline tickets versus which card needs to be used to book car hire or hotels, dependent upon the triggering clauses of the various cards is a minefield for me.
Reading, remembering and being able to separate in your mind the various PDS's is also troublesome. I could see myself easily forgetting that the card in question isn't covering domestic car hire (for example) as this is a really odd exclusion.
To truly suck the juice out of cards "free" insurance you'd need, I think, to limit yourself to one card only (or be prepared to use only one cards insurance at any rate) or you'd need to be one of those lucky folks with an eidetic memory. So far I've not found a single card insurance which is as complete, in an overall and well rounded way, as even fairly cheap dedicated travel insurance policies, and as I don't have a great memory, trying to link together several cards insurance to provide full coverage appears to be beyond me