Total BS. This guy really doesn't know what he is talking about. In general, the added costs can be made to be worth it if you do oil analysis and extend the change intervals. As for it causing more problems, the only times I have seen synthetic fluids cause problems were in cases where it was a very old car gunked up with old fluid, and the synthetic fluid revealed leaks that had been stopped up by the gunked up fluid.
That about sums it up! There is tons of data on synthetic performing well in n/a Subaru's. Some up to 12,000 miles on the oil, guided by used oil analysis. There's actually a sticky up top (Blackstone).