I have heard for quite a while about mule deer not surviving in the east. I have had a small herd in south central PA for a couple of years. They are pure mule deer and have had few problems. A coupe of disease issues have caused losses, but once we learned those, those issues are easy to manage. The temperament of mule deer is quite different and they are a real joy to be around and handle.
I also have mule deer in south central Oklahoma. Again we have seen the same disease issues, but have been able to control them.
As far as cross breeding, I am not a fan. We had a couple in Oklahoma, and lost one to EHD this past year. They may be somewhat resistant to EHD, but not as resistant as my pure mule deer, of which we have not seen a single loss. In the northern area, like ND and MT they saw heavy losses of whitetail in some areas, but it did not appear to affect the mule deer. Additionally if would be a shame to dilute or pollute the antler characteristics or temperament of the pure mule deer. Much as it had been with elk.red deer hybrids.
Just my thoughts, but still a lot to learn
Ray Favero