They are marked as unsafe because there are hundreds and hundreds of intrinsics, some of which do memory access, some have side effects and others are arithmetic only. Someone would have to individually review them and explicitly mark the safe ones.

There was a bug open about it and the rationale was that no one with the expertise (some of these are quite arcane) was stepping up to do it. (edit: other comments in this thread suggest that this effort is now underway and first changes were committed a few weeks ago)

You can do safe SIMD using std::simd but it is nightly only at this point.