If your DEM is good, Terrain Flattening is able to reduce radiometric distortions at slopes. Areas facing towards the sensor produce higher backscatter than areas facing away from the sensor. Terrain Flattening is able to correc that.
Both Range Doppler Terrain Correction and SAR Simulation Terrain Correction do the same: They correct for geometric distortions caused by topography. But they cannot restore information pixels affected by layover, foreshortening or shadow.
The difference is that SAR Simulation allows to output the potential shadow areas.
I made a comparison of S1 products processed with and without Terrain Flattening: S1 radiometric correction
Nice summary by JAXA: https://asf.alaska.edu/data-sets/derived-data-sets/alos-palsar-rtc/alos-palsar-radiometric-terrain-correction/