Macintosh bootable volumes are supported on the internal 2-port PCI card (part number TSATA) but are not supported on any of the other SATA cards at this time.
Often, yes. Sometimes, not. Some other manufacturers' drivers may incorrectly assume a Sonnet card is their SATA card which may cause havoc. The symptoms include significantly slower performance, driver not loading, or both drivers loading and causing and kernel panics as they both try to access the same card. Sonnet recommends removing other manufacturer's card(s) and driver(s) if a problem of this type occurs.
Unfortunately, not. The driver was written by the 3Gb SATA controller chip maker, Silicon Image, and they no longer maintain the macOS driver for this chip.