The only ones with which I've had success were the older ones, with ext sty.

1. Convert the Style from SFF2 to SFF1 format.
2. Next use Jorgens Old Format Converter to remove the Mega voices in the style.
3. Only now convert it to Korg.

I did it for you. Cannot test - feel free to try it.

That's great Henni!
Please educate me... Is the main difference between SFF2 and SFF1 tracks that contain NTT type = Guitar?

Yes, that's right. Those do not convert easily hence the process. Yamaha styles are the absolute most difficult to convert to other brands.