Asynchronous engine uses an external tool that pings IP addresses in batches of 300 using the asynchronous ICMP requests, as its name states. The result is rather fast, but not always 100% reliable.
Nmap engine uses external nmap.exe tool that pings IP addresses by its own logic. Pretty fast as well.
Classic engine uses internal technique with creating 20 threads (by default) where each thread pings one asset at a time. It's rather slow, but more reliable, as we've found out. It's selected by default.
Actually you can try them all and see which one suits you best.
Please explain your second question, as I'm not sure what do you mean by full scan, and when do you get no username.