But on the web, you obviously can't tolerate true unsafe back doors. This project gives you something that is "unsafe" in that you can read or write to parts of the allocated "heap" buffer that you aren't supposed to, but it's not possible for that to violate JavaScript's safety. And the compiler is generating extremely optimizable code, so it JITs very, very well.
On top of that, the fact that it's all implemented on top of a simple API (typed arrays) means that you could compile in a safe mode that does extra checks, or even trap all memory traffic and build valgrind-like diagnostic tools. It's starting out as a virtualized abstract machine, which means it's easy to hook into and provide great development tools.
I probably wouldn't reach for this kind of dialect for most purposes. But for very performance-sensitive kernels, it's got promise. But it's an experiment, so we'll see!