I have worked on both a major JS runtime and a .NET runtime. I have fixed hundreds of security bugs in JSC. Based on that, I have no doubt that what Fil-C does is safer. Just the absence of a JIT makes it safer in a way that I don’t think is seriously debatable. Even with JIT disabled, a JS runtime has massive attack surface due to the language relying on a large native library to do anything useful, not to mention a mind boggling amount of language implementation corner cases.

By contrast Fil-C has a small number of rules and largely obviates the need for “native” code.