Since the latest version of Polyphone (which can read FLAC-files as audio samples) many SFZ-instruments can be read with Polyphone. That's why I personally converted several soundfonts to SFZ, because then I have a format which is readable with Polyphone but also with applications for SFZ-files. (SForZando can import soundfonts, though than the samples are stored within one big consolidated audio file and it can happen frequently it doesn't sound like it's supposed to).
The troubles appear when soundfonts contain modulators. Or vice versa when the SFZ-files contain opcodes about behaviour which is not included in the soundfont specs. (
Here are those specs by the way. Good luck by reading it hahaha!)