There is actually an significant technical issue with user-supplied microcode on modern fast CPUs: the control store is not fully writable and only small parts

There is actually an significant technical issue with user-supplied microcode on modern fast CPUs: the control store is not fully writable and only small parts of it can be patched in the field. Reason for that is that the control store is probably the fastest block of memory in modern PC as it has to have latency on the same order as CPU clock period and usually has very wide data bus. Implementing this as entirely writable RAM would be uneconomical.