I agree that for many people interest only mortgages are a disaster waiting to happen, but not for everyone. You need to:
have a very healthy loan to value ratio,
buy sensibly; preferably a low maintenance property,
choose an areas where the rental yield is healthy and the market is solid and proven with a low risk of voids,
have done your maths and your due diligence
be in it for the long term,
be confident that you have the funds to weather any sudden interest rises that might push you into a cash-neutral situation each month or even a cash negative situation for a while.
The trick is to be able to keep hold of the house and keep things ticking over without accumulating debt throughout a housing crash and/or periods of rising interest rates. If you've kept somewhere for 15 years or so and need to sell it to repay the interest only mortgage then providing you had a decent amount of equity in there to start with, and you didn't choose your mortgage very unwisely you should have not only made money on the house but also on the rental income. The people who really come unstuck are the ones who are forced to sell in a downturn or with negative equity. that could happen to any of us at any time if we've owned them for ten years or less, (give or take, as a rule) but the key is to HAVING A CHOICE about whether/when you sell up. Even if there was some unforeseen huge financial disaster and you found you'd slipped into negative equity for a few years, providing the rental income is paying the mortgage or at least just covering costs then eventually it will right itself. Just stick with it. But obviously you can only do that if you have a healthy cash float for emergencies.
The people who come unstuck are the ones who go into BTL by borrowing in their own home, very small deposits, poorly chosen borrowing, no plan for future downturns or rates rises, poor due diligence and rose-tinted glasses. Also leveraging too highly and too quickly can be disastrous as we now all know. But after the spectacular crash and burn stories of the last decade we've all heard it's actually very difficult to borrow to that extent now anyway. Almost impossible in fact.
The days of making money thick and fast from BTL are well and truly over, but hopefully so are the days of losing it! We can all learn from the people who have been there and done that, for better or for worse.
These days there is still long term security and money to be made in BTL but you need to be well-versed in all the things that can go wrong, and have contingency plans. You need to be in it for the very long haul, too. nothing can be flipped for quick money any more - not unless you REALLY know what you are doing and even then it's very risky.