Is GDP adjusted for PPP a good or a bad comparison of the economies of two different countries?
Math is a highly inaccurate proxy of the state of economics as is the sum of goals scored in soccer an unreliable proxy of the quality of the game played.
Two countries with the same GDP will have different economic performance, just as two games of soccer with the same score will have different gameplay. Economics is not about numbers, economics is about the enablement of the gameplay of freedom and how that gameplay rejuvenates opportunity to yield a reliable proxy of future performance.
It doesn’t quite matter what type of math you use. So to answer your question, GDP is a bad proxy of economic performance from which no pertinent conclusions should be drawn.