Magnesium hubs dissipate heat quicker, therefore, in conditions of high atmospheric temperature, this is certainly a positive factor. The hub is in direct contact with the rim, therefore, if it works at lower temperatures, it helps the rim to dissipate the heat generated by the tire. Temperature is important in managing the tires on the track, as a correct pressure ensures better performance. Vice versa, in the case of lower atmospheric temperatures, the use of aluminum hubs may be more suitable which, combined with rims made of the same material, help the tire to reach the optimal operating temperature much faster.
Each kart manufacturer has different hub lengths to choose from. Usually, the standard hub supplied at the time of purchase is the ideal length to make the most of the chassis, on average, in any type of conditions.
As a general indication, we can say that the longer the hubs are, the more they stiffen the kart setup.