Made some more tests with the following results:
1. The converter needs a 15kHz (interlaced) sync signal, VGA standard 31kHz (progressive scan) won't work.
2. (Near) standard resolution and framerate recommended (576i@50Hz or 480i/240p@60Hz).
3. The PAL-NTSC switch must be set manually to the appropriate video standard of the input signal, because the converter doesn't autodetect it, and cannot make a flawless conversion between them.
The following cores work well in scandoubler disabled mode, converted to composite:
Acorn Electron, Amstrad CPC, Apogee BK-01, Atari 800, Atari ST, BC Micro, C16/Plus4, C64, Didaktik family, Dummy/Menu, Elektronika BK0011M, HT1080z, NES, Primo, Sam Coupé, SMS, Sinclair QL, Specialist MX, Vector 06C, VIC-20, Videoton TVC, ZX Spectrum.
Atari 800, Atari 5200, Bally Astrocade, Chip8/Superchip, Colecovision, C16/Plus4, C64, PET 2001, MSX, SMS, Genesis.
These cores have problems:
Amiga/minimg: oversaturated but generally dark picture, can be (partially) compensated by adjusting the trimmers of the converter.
Atari 2600 and PC Engine cores give a Black&White picture. Sinclair ZX-81 produces a vibration.
Cores with no SDTV standard refresh rates/resolution won't work (i.e. Macintosh, GameBoy).
Didn't test the Arcade cores, but all of them with standard resolutions and refresh rates should work, if they are RGB (15kHz) capable.