The main reason for using helical gears is the increased number of teeth in contact at any given time, which is a fundamental requirement for smooth torque transmission. With their increased contact ratio compared to spur gears, helical gears have a lower fluctuation of the gear mesh stiffness.
In planetary gearboxes, however, it's much more difficult to design around these axial forces for two related reasons. First, there is typically very little room in a planetary gearbox to incorporate the kind of bulky bearings that can tolerate high axial forces.
Second, the planet gear bearings need to play an active role in torque transfer. Planetary systems split the torque input from the sun gear amongst the planet gears, which in turn transfer torque to a planet carrier connected to the gearbox output. The bearings that support the planets on the carrier have to bear the full brunt of that torque transfer.
the best advice on gearbox selection is to favor helical planetary gearboxes in noise-critical applications such as medical equipment, lab automation systems, and printing machinery. When low noise is a primary concern, you can often upsize a helical gearbox to reach the desired torque and lifecycle requirements.
But upsizing the gearbox drives cost. So if noise doesn't matter as much, you'll be better off with a spur gear planetary gearbox that will deliver higher load capacity and longer life in a smaller package.