When you recompress video from lossless to lossy video, of course quality will decrease, because it lossy codecs.
In lossless mode action (High quality settings) quality better than DXtory YV24 (high).
Here comparsion:
DXTory YV 24 vs Action:
http://screenshotcomparison.com/comparison/100574
Action has much smoother gradients and better quality.
DXTory RGB(True quality) vs Action (High quality):
http://screenshotcomparison.com/comparison/100580
Only slightly worse than RGB(almost uncompressed video), because fic is not RGB and has higher compression.
With RGB, load on the processor/video card/hdd is much much stronger and much less performance in games.
Where is the poor quality?, slightly worse than the RGB?, this is bad quality?, is almost perfect quality with a much smaller size.
After recompression of the original format, the further the quality depends on the codec and settings that the user sets, the entire responsibility for the further quality depends on the user, not from mirillis action.