It's not a safe compiler for C/C++, the compiled wasm code can still be compromised, it just cannot touch the rest of the process except indirectly via returned values.
The big difference is that the WASM sandbox significantly reduces the surface area of what bad stuff can be done.
Today, a compromise in the browser means the attacker can do whatever the browser can do (which is usually a LOT). With the sandbox, a compromise can only really affect what the sandbox has available to it. That means, if your sandbox only exposes a single method which takes in a string and returns a string, the worst thing an attacker can do is return a malformed string.
Of course, if you mishandle that returned string then bad stuff will happen but it's a far cry from the input string being able to potentially cause arbitrary code execution which installs a virus on your machine.
To really do something evil you have to not only compromise the code running in WASM, you have to find a way to break out of WASM. That's a lot harder to do.