You can play soundfonts. However, if you run out of polyphony it just stops working. Most samplers including the Audigy and Soundblaster chips intelligently cut off the oldest voice. Not the X-fi. Thus, the usable polyphony is actually better with the older cards.
Actually, I don't think you're right about the SBAudigy. When it came out, there were loud complaints of polyphony problems because the drivers cut off the oldest voice, which could easily be a held foreground note. It was changed to be like the old SBLive drivers, which were based on cutting off the highest-numbered active midi channel, which gives the soundfont composer more ability to control things by assigning the most important instruments to the lowest-numbered channels.
You can use the ASIO drivers. With the X-fi you can set them as low as 1ms latency and they work.
However, if you try to use soundfonts at the same time the ASIO drivers will crash. Even with latency set to 100ms or more. This makes it pretty much useless for recording if you thought you were going to be able to use Soundfonts.
You appear to be assuming that this is happening to anyone who tries it because it's happening to you. Are you sure? I watch this group pretty closely, and it seems doubtful I wouldn't have heard about this previously here if something that blatent was wrong that wasn't actually caused by either system corruption or side effects of a bug in something else entirely that's installed in your particular computer.