It sounds like your main reason for looking at DVC is to make staying at the nicer places more affordable. If that's the case, then the finance charges are going to blow that cost savings away. Disney's finance charges are extremely high. Also, if you plan to buy direct from Disney, you'll potentially be paying twice as much as you would for a resale contract, which again will cut into any money you'd be saving on your stays, big time.
To answer your question about why it's a bad idea to buy with the intent to frequently use points on cruises or non-DVC resorts, I'll break it into two parts:
1) Using your points for
DCL is not part of your DVC ownership. By that I mean that it's not like using your points at a DVC resort; it's a perk that's currently available, but can change at any time. DCL restricts the number of cabins and cruises that members can book on points, so you might find that the cruise you want to take isn't available unless you pay cash. And by the time you figure in the high initial cost of buying DVC from Disney, the finance charges, and the annual maintenance fees, you may well be paying more for that cruise with points that if you just paid cash for it. Also, 200 points might get one member of your family on a DCL cruise, but no way will it cover all 5. The point costs for the cruises are very high.
2) When you say you want to use points for resorts out of the country, that means exchanging your DVC points for another timeshare. You can buy a reputable, big name timeshare contract at resale for a few dollars that will let you exchange into those very same resorts that DVC offers. Why buy a $29,000 timeshare to exchange for international resorts when you can do the same thing for a few bucks?
The bottom line is, DVC isn't a one size fits all vacation solution. You can use it that way if you want to, but there are way less expensive and less restrictive and less complicated ways to accomplish the same thing.
DVC can be a great value if you use it to stay at the DVC resorts. Look at the resale market, and look at a smaller contract that will cover your stays at WDW and DL and Aulani. You might find that you can afford that without financing.