It is only the filling of a standard TIFF description tag in a structured way with date, gain, exposure time, sensor temperature, used filter.....
Standard libraries like www.libtiff.org support the tag Description. The Description is just a string you could fill with the FITS header except that each line is completed with a LF or CR+LF as you like. LibbTiff has only LZW compression which performs a little less then Deflate. (LZW is used in GIF, Deflate in PNG)Personally I'm use a Pascal library. Code for importing was simple since the FITS header is of simple structure. Filling the Description tag was only a few lines of code.For Ekos mainly writing Astro-TIFF is interesting. With the compression method Deflate (same like ZIP) achieves almost the standard FITS compression factor called RICE from the CFITSIO utilities FPACK and FUNPACK leaving about 60% of the file size.
TIFF is super flexible format. I don't think there are any compatibility problem since it is already a few decennia’s available. Except for FITS it is the only generic format which allows 32 and 64 bit in floating point images. But the intended purpose if for 16 bit lights, darks, flat and flat-darks. Astro imagers produce these files in the thousands. The files size reduces to about 60% and you can use standard image managers to organise and view your images. So the intention is just use the power of this existing established TIFF format.
Siril version 1.0 to be released soon will accept Astro-TIFF for stacking. CCDCiel has implemented Astro-TIFF reading and writing fully in the code. The next ASTAP to be released today or tomorrow will fully support it including batch conversion both ways.