PirongiaPete;
Every drive type has its advantages and dissavantages. Harmonic drives have big ratios in one step, but are not very stiff, and somwhat inefficent. Harmonic drives are typically used for low HP service, often with servo motors. Ordinary, parallel shaft drives have efficency of 99% per stage. For 20:1, you need 2 stages and get 98% efficency.
Parallel shaft drives are a safe first choice. For special constraints, and ecconomics, other types may be better.
Multi-path systems, such as planetaries and split power path drives have higher power density and make more efficent use of mass and space.
There are always new ideas comming out, but nothing currently happening to sweep choices in a new direction.