Multi vs Single Layer RF Coil
Single layer coils are easier to build.

Multi-layer coils are more compact for the inductance, however generally have a higher self capacitance, so lower self resonant frequency (SRF).

Taken together, that means if you are instructing amateurs to build something that will work first time, specify single layer coils. If you are a manufacturer with time to design properly and an incentive to minimise use of materials, use multi-layer coils.