Ideally GA does seem to have discrepancy of about ~30%, which means your client should be able to track 70% of the traffic. If your advertiser is facing issues with GA reports, it is sometimes due to traffic delivery or synchronization frequency. The best way would be to obtain site reports from your advertisers(GA report) and manually block sites with highest discrepancies. This result can be obtained after running the campaign for 2-3 days and can help with the reportings. MSID Dynamic parameter tracking allow you to block low converting soruces.