As far as lifetime and wear are concerned, in-line planetary systems distribute the load remarkably well among major components, and the economic size is evidence of this. If all the components were of similar quality, yet a potential weak spot had to be singled out, it might be the bearings that support the individual planet gears.
Here space is often very limited, so planet bearings can be small compared to some ordinary gear-and-pinion reducers where there is a lot of leeway for larger bearings. And don’t forget, the canceling effect of multiple planets on radial loads applies only along the central shafting; it is in fact the radial loading on individual planet bearings that drives the carrier around.
Thermal and cyclic fatigue may be promoted in these bearings by the limited load distribution and the fact that planet gears can spin pretty fast on their axes. What’s more, with high speeds and heavy planet gears, centrifugal forces can add significantly to the burden. That is not to say that planet bearings don’t often outlast other components. And clearly, high-grade precision bearings used with low-grade high-tolerance gears isn’t an “all things equal” arrangement.