The load taken up by the planets is, in real situations, not perfectly balanced. One planet might by chance end up radially closer or further than the others from the sun axis, or the axis of the carrier rotation might be slightly off. As the precision of manufacture goes down, and the number of planets goes up, the tendency for imbalance increases.
Sometimes the effect of an imbalance is small, and the operation is able to accept it. The planets might even “wear in” and gradually engage the load more evenly. But some designs will be sensitive even to slight imbalances, and may require high-precision components and assemblies; pinpointing the proper locations of planet pins around the axis of the sun gear could be the key.
Other schemes that help balance planet loads include using floating subassemblies or “soft” mounts to allow some small radial movement of the sun or the planet carrier. In this way components can continually shift just a little, helping to equalize uneven loading. Mounting part of the assembly soft is not an uncommon design practice, but stiff assemblies can have advantages over flexible ones, so the answer isn’t always cutand- dry.