How much is the relevance of a synthetic benchmark?
Having just a synthetic benchmark is generally weaker than having a realistic benchmark, but better than nothing. 

There some more considerations to be taken into account.

First, quality considerations: Is the design rationale of the benchmark clearly communicated? Was there any effort made to reflect realistic cases? Was the benchmark designed with input from multiple, independent experts (ideally different authors than the author of the paper, due to researcher bias)? Have independent experts been consulted to validate the benchmark? Is the benchmark publicly available and well-documented?

Second, historical considerations: Has the benchmark recently been used in other research? The more widely a benchmark is used, the better, since it will allow an easy comparison between different works.