No ICO can offer 100% guarantees on a ROI (return of investment). But the upside of this ICO is that there is a guarantee that the token can only drop so far, because it's backed by real world properties. REAL have mentioned in their white paper that they estimate 10-12% profit on your investment. Not a big increase compared to some ICO's, but the big part is stability. As the saying goes, slow and steady wins the race.
10-12% could be seen as a conservative estimate. The housing market is doing well and when properties are sold you will get parts of the profit (if you used your REAL tokens to invest in said property).