Before revision:  This explains why a 2D classification approach based on multi-reference alignment (MRA) is not as popular as the Bayesian-based approach despite that the former has advantage in differentiating subtle structural variations under low signal-to-noise ratio (SNR) . This is perhaps because that MRA is a time-consuming process and a modular GPU-acceleration package for MRA is still lacking.
After revision:  This explains why a 2D classification approach based on multi-reference alignment (MRA) is not as popular as the Bayesian-based approach despite that the former has advantage in differentiating subtle structural variations under low signal-to-noise ratio . This is perhaps because that MRA is a time-consuming process and a modular GPU-acceleration package for MRA is still lacking.