What are some concrete (near-)"worst-case" examples of subset-sum?
The naive dynamic programming complexity is $O(sN)$ where $s$ is the integer bit precision, $N$ is the set cardinality. With some slight tricks in memoization cutting the precision constant in half, an instance with 64 bits precision should be solvable without much optimization in a linearly increasing fashion and as fast as memory access allows.

However, the naive method for small $s$ but large $N$ is dynamic programming which has subjectively large memory requirements. In practice this is the constraint that is prohibative for large instances skewed towards small precision, since many of the non-cryptographic applications I can think of deal with embedded systems having low memory and real-time constraints that limit non-uniform memory access.

At the other end of the spectrum if you have PiB subset-sum instances with small $s$ but very large $N$, the same issues arise as in sorting. These instances are incredibly 'easy' from a theory perspective but the micromanagement constant factor details dominate such as cache usage, memory walls, parallelization, bandwidth saturation, and code/data locality. Many of the techniques for fast solvers become invalidated at that size due to either reliance on a central memoization table or chaotic memory access.

The major applications for subset-sum that I know of are cryptographic in nature, which necessitates that the natural 'real-world' integer precision is far larger than 64 bits. In addition, most of the various manufacturing and finance applications do encounter the need for integer precision much larger than 64 bits as a way to actually encode the problem without blowing up $N$ in a very non-linear manner. This is required even though the original 'real-world' quantities are far less than 64 bits.

So hard subset-sum instances with 64 bit precision don't exist in the manner that I think you want to test. If an analogy helps, this is similar a complexity outcome to finding the prime factorization of a very large number but limited to prime factors with 64 bits of precision rather than the general case.