Regarding conversions of integers, C 2011 [draft N1570] 6.3.1.3 2 says
Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.
Thus, converting -1 to an unsigned integer type necessarily produces the maximum value of that type.
There may be issues with using -1 in various contexts where it is not immediately converted to the desired type. If it is immediately converted to the desired unsigned integer type, as by assignment or explicit conversion, then the result is clear. However, if it is a part of an expression, its type is int, and it behaves like an int until converted. In contrast, UINT_MAX has the type unsigned int, so it behaves like an unsigned int.
As chux points out in a comment, USHRT_MAX effectively has a type of int, so even the named limits are not fully safe from type issues.