I haven't seen a rental guarantee where the actual net amount of money you get will cover your mortgage payments, what to say of putting you in positive cash flow.
The rental guarantees are marketing tool to attract the buyers or to differentiate from other similar projects.
The facts show that most of people who can do some basic maths don't take rental guarantee.
There are few ways that developers will arrange for that guarantee:
1. By bringing property management company of a hotel operator on board
This is the most genuine guarantee, but you have to have on thing in mind - they will only take a risk to give such a guarantee if there is a big margin left for them. That means that you will eventually be putting your self out of pocket, you could be renting it your self for (much) more then the guarantee is. If you are in he place like Dubai where the rents are rising very rapidly, it probably wouldn't be a good idea. If the property is connected with the hotel, then is is quite usual that the guarantee will be for at least 5 yrs. the reason is that in principle for the first year or two the operator will be loosing money on it (unless you are at the great location),but then it will pick up and they will make good money on the next 3 yrs. In any case, they will not give it unless there is a good financial safety buffer.
2. By building in that amount in the price of the unit
This the most common and the most deceiving guarantee. The developer will come with the end price for the project, and then will ad some percentage on the top as the rental guarantee. It doesn't at all reflect the facts on how rentable this property actually is.
3. Insurance
The developer will get some insurance company to give guarantee. This is very similar to the first one, the insurance company will perform all the due diligence and will only give it if their rental projections are around 50% over the guarantee, so they can have some safety buffer as well.
The first step when you see that kind of guarantee is to ask the developer for the guarantee document, and unless all the details are there, find who is issuing the guarantee.
If the guarantee is by a property management company or an insurance company, then you know that is a good property and that you should NOT take the guarantee because you can probably make 50% more by renting it your self. But it indicates that the property is a good investment.
If it's by the developer, then you can totally disregard it and due your own due diligence, and then make a decision after you have assessed the rental potential. The best way of doing it is to go to 2 local letting agents. They will give you the best picture.
And all of this is only if the numbers stuck AFTER getting a NET return figure by deducting all the charges etc.
8-10 yrs of 15% guarantee in Ajman sounds crazy, and little digging will soon reveal some interesting facts.