They have some cool ideas, but many are incompatible with existing software, e.g. all the memory protection stuff. Other ideas like how memory loads work could be integrated (and have been in place in GPUs for a long time, though not in CPUs) but still depend on compiler changes, which presumably they've done, but past experience with hardware projects that rely on compiler changes is that the compiler changes aren't performing well enough in practice. Or perhaps compilers improve so slowly that traditional architectures can easily absorb the ideas in time.
Another example is that Apple's M1 proves that wider frontends are possible in traditional architectures, as long as the instruction size is fixed. There's no particular reason to believe that the Mill would be inherently better at this than a traditional architecture.
If you have a lay person's interest in hardware architecture and haven't looked at their docs yet, do so. The ideas will likely tickle your brain in a good way -- they did for me. But don't expect much, if anything, from them in production.