ALSA is an API for hardware devices. That's root to a lot of problems you encounter when using the PA plugin for libasound. There are certain differences between hardware and virtual devices. Timing works differently, memory management as well.
The devil lies in the details. It's a lot of small issues that make compat difficult. In addition, some fundamental parts, like the reliance on the classic period-based playback model is a weakness that libsydney does not expose. Not exposing periods will greatly increase compat with glitch-free and with network setups. Finally, libsydney allows clients to set all kinds of properties for a stream that make it especially useful for desktop sound servers. (i.e. setting icons for a stream, and so on...)