Very few Eurorack modules actually use +5 V, and few of Sam’s do too. Those that do, like the 1222 VCO, use a 5 V regulator connected to the +12 V rail. If you plan on making a lot of use of microcontrollers etc. that need 5 V or 3.3 V, you might want to choose differently.
I don’t know why Sam went with the Eurorack style ribbon cables for power. It’s good because you can leverage Eurorack designs and equipment, not just bus boards but module circuit boards; you can fairly easily adapt DIY Eurorack modules into Kosmo format. On the other hand it’s widely agreed that ribbon cables are not the best choice for power distribution. Fewer larger conductors makes more sense. The MOTM and Frac Rack formats use 4-pin MTA-156 connectors for ±15 V and two ground wires (ground has to carry as much current as both voltages), as do some 5U formats, while others use a 6 pin MTA-100 for ±15 V and +5 V.
It’s your choice. Use what appeals to you, and is technically suitable. Eurorack will give you lots of compatibility options, others may perform better and give you access to more voltages. Nothing seriously wrong with either.