As far as I know, Solaris threads are actually M:N threads on the system level. M:N threads are extremely efficient. A "native" thread is a heavyweight construct - e.g. by the stack space allocation. So this puts a limit on how many threads you can spawn in a program. For many tasks, this would force you to roll out your own M:N mapping system yourself. Doing it on the language level usually yields the better results.

Due to the implicit M:N mapping, Go goroutines are extremely cheap. This allows you to spawn as many, as your algorithm naturally requires. The Go runtime will automatically map them to native threads - typically one per avaliable CPU. As a consequence, a Go program that heavily uses goroutines has a pretty clean code and scales without much overhead across a large variation of number of CPU cores.