Why is it "harder"? I assume you mean harder to quantify than vaccination immunity and I don't believe that's the case, but I'm curious why you believe that is the case.
edit: surprising downvotes so I probably didn't communicate my point well.
I can't imagine a reason why natural immunity would be harder to quantify than vaccine induced immunity. It would seem to me the methodology to quantify both would be very similar.
With a vaccine, you get a standardised dosis of an agent resembling the disease or just a part of it. The remaining variability is the response to that standardised input.
With an infection, you have no control over the degree of exposure, so on top of the variability of the response comes the variability of the exposure.
Additionally, in case of exposure of the full pathogen, there is some variability to what part of the pathogen the immune system will respond. That part can actually be a good thing in total.
we are testing for antibodies but antibodies aren't the only immune system response and depending on who you ask might not even be the "main" defense against covid.
edit: surprising downvotes so I probably didn't communicate my point well.
I can't imagine a reason why natural immunity would be harder to quantify than vaccine induced immunity. It would seem to me the methodology to quantify both would be very similar.