1. You'll still have to come up with a lump sum (essentially a down payment) or "option" to enter into the agreement. This is due upfront and is usually a percentage of the purchase price. Plan on $3000-5000.
2. You'll agree on a date when you'll have to exercise the option to buy. Let's say 2 years from now. At that time, you'll still have to qualify for a loan, and have sufficient down payment.
3. If you pay the rent late even once, the seller typically reserves the right to keep everything and void the deal.
4. How do you know the price you negotiate down the road will still be a good deal when it comes time to actually purchase the property? Will you qualify for the financing then? Will it appraise out?
5. You'll be responsible for the upkeep of the property throughout the life of the lease. If the furnace goes out, you'll have to fix it. Same for the roof, plumbing, and everything else. Remember, this is for all intents and purposes now "your" house.
Violate any of the above terms and the seller keeps everything you gave them upfront, and also all of the money credited towards your down payment from each month.
Too much risk, too much money, and too much uncertainty about what your situation or finances will be too far down the road in the future. Please ensure you do your homework and are aware of the terms of these deals. There is NO deal out there that allows you to simply rent a home until it's paid off - without having your credit, income, and finances factored in & considered.
Hope this helps!