The downside of means testing pension payments is that it acts as a disincentive to discretionary pension saving.
The previous UK system (pre-2009) was an extreme example of this where those without additional pension savings got topped up with pension credit (GIS equivalent) that was clawed back at 100%. They ended up raising the state pension (OAS equivalent) to that level so those with a full contribution history are no longer eligible for pension credit (12,548 GBP = 23,416 CAD).
The OAS clawback rate is only 15% which seems more like a tax on pensioners - reasonable given they no longer make social security contributions though maybe the 50% GIS clawback would disincentivise workplace tax-deferred pension saving for those affected.