Maltz and Targonski  have examined the county level
UCR statistics used by Lott and found it to have extensive gaps--in
many states large numbers of counties have 30% or more of their
population missing coverage. They conclude that
in their current condition, county level UCR crime statistics cannot
be used for evaluating the effects of changes in policy.
I should note,
however, that state level analyses do not have the same problem and
these give somewhat similar results to the county level one.