Usually cost synergies are easier to predict because they involve cutting a specific expense. For example, after an acquisition, there is no need for 2 headquarters, so they may shut down one headquarters and save on rent expense. The rent expense would be relatively easy to identify and quantify.
Revenue synergies are less tangible since it is harder to guess the exact increase in revenue as a result of synergies; it’s usually much more of an estimate. For example, it would be difficult to estimate the number of products that would could be cross-sold to an expanded customer base. A lot of assumptions would have to be made and the projection may not be as accurate.