The engine in t/o climb is operating in a high power regime - perhaps, initially 110% of cont rated power, reducing to 100% for the climb - this is
obviously fairly stressful.
In cruise, the engines are loafing usually, obtaining maximum range for a given fuel burn and reasonable duration. A typical thrust setting for high altitude cruise would be around 70% of cont rated power. This is often at or near V (md). Any corrections for speed/climbrate are going to be small, and are not time-critical.
On descent first the power is retarded to reduce speed and lose altitude - it is usually flown at less than V(md) in order to take advantage of the higher drag at lower speeds, and to shorten the distance needed to let down (cabin re-pressurisation requires low descent rates). Thrust % may reduce from 25% initially to 13% just outside the TMA. This is more stressful than it sounds - the relatively cool air, and low power can cause problems with either icing or surging.
But worse is yet to come
- on finals, the reconfiguration of surfaces now forces power back to around 60% to hold the approach path and speed, and also calls for power adjustments to follow the approach angle (remember pitch=speed, power=descent rate during the approach, and small deviations are all that are allowed without a missed approach & go-around). With correction for wind shear, or go-around full power may be commanded in a very short time.
Once you also include FOD damage (from runway debris, or bird strikes - the bird populations are much denser near ground level, particularly near lakes/marshland/coast which also make the most common sites for airfields) then the stress/risk on engines, pilots and airframes is at its worst near the ground, both on the way up, and on the way down.
