How to Merge Multiple MP4 videofiles using concat of ffmpeg while transposing 180 degrees two MP4 videofiles
I finally found out how to do it! 

ffmpeg -f concat -safe 0 -i VIDEOFILES.txt -filter_complex  \
 "[0:v] fps=32, scale=1920:1080, vflip, hflip, setpts=0.25*PTS, fifo [v0]; \
  [0:v] fps=32, scale=1920:1080, vflip, hflip, setpts=0.25*PTS, fifo [v1];  
  [v0][v1]concat=n=2:v=1[out]" -map "[out]" -crf 14  -vcodec libx264  -vb 20M  -loglevel error  video_out_01.avi



concat: concetrate files 
VIDEOFILES.txt: video files with path
included, eg. file '/path/a.mp4' (including file) 
fps=32, scale=1920:1080: drop to 32 frames, and reducing video resolution
vflip, hflip: 180 degrees rotation of the video  
setpts=0.25*PTS: Four times speeder  
fifo: a ffmpeg filter that it says to wait frame
to be fully processed before it is given to the next process --- due
to some filter process frame quicker than others e.g. due to frame
drop (?)
crf 14: High quality ensured (0 = uncompressed)
vcodec libx264: codec to compress the video file
vb 20M: video bitrate (a high one)
loglevel error: ffmpeg output information only errors will be displayed


BIG Questions:

A) I tried to reverse the order of filters eg. 
[0:v] setpts=0.25*PTS, fps=32, scale=1920:1080, vflip, hflip,  fifo [v0]
However, it was extremely low. 
The commands are executed seriasly? Then, why I must recode every frame, when only 1/4 of them will be kept? How can I do that in order to gain recoding speed? 

B) Why I need FIFO eventually. This must be not the default behavior? All frames that are kept must be processed from all filters! Right? Why then ffmpeg drop frames without being processed? this is the meaning of FIFO? Right? 

UPDATE: ffmpeg 4.2.3 freezes when multiple video files exist e.g. >5