Quantum Chromodynamics, path integrals, and what other mechanisms you need for deriving the half life are extremely complicated, especially for that many particles, so of course it's hopeless without approximations, which can be wrong, or are even wrong by definition. But the fact that it's prohibitively hard to solve the equations for such complicated systems doesn't indicate a gap in our understanding. You don't even have to go to xenon, this is already the case for tritium.