The toughest operating regime for a/c tyres is a long, fast taxi at MTOW. The sidewall flex heats the tyres to very high temps. As evidence, just look at all the failures on t/o ex HNL reef runway (08R). I wonder about some pilots who want to taxi from the 120 gates to 25R at LAX at 30kts on a B744 at 397 tons!!
As has been stated, touchdown is not too tough on tyres because the a/c weight is still largely supported by the wings.