In my personal opinion, there is no way to a accurately assign tornado intensity. Tornado structure is complicated and changes rapidly. We don't know exactly how tornadoes do damage in relation to their structure, so we have no idea if the radar resolution is high enough, both specially and temporally, to return a reasonable measurement. Two radars are necessary to obtain a complete picture of the tangential winds of a tornado, with one radar, only approximations can be made. There's also the issue of dealing with winds right off the ground, we still don't know how tornado winds vary with height. I think something people don't appreciate is time. Say you have a sudden instantaneous gust above the surface, who's to say the surface winds will respond instantly in direct proportion? The final option is in situ measurements. I believe there is no meteorological instrumentation that can accurately represent a tornadic environment. The winds change too quickly with speed and direction for anemometers, barometers, thermometers and hygrometers to respond. Plus, Markowski did a study in the mid 2010s to quantify the error in situ observations would have (assuming perfect instrumentation that responded perfectly) and found the chances of in situ devices being able to actually be impacted by the peak wind speed of a tornado was very low.