Fusion: Turbulence, plus Naiver-Stokes was known to be hard, plus adding in EM fields, plus the exothermic state change of the nuclei fusing introduces discontinuous jumps in the free energy.
Collatz: if you replace 1 and 3 with arbitrary parameters, Conway showed that this class of problems is undecidable, so any particular instance could be arbitrarily hard.
Collatz: if you replace 1 and 3 with arbitrary parameters, Conway showed that this class of problems is undecidable, so any particular instance could be arbitrarily hard.