Drive Train Gearboxes

[thinker&planner]

Background: We have had CAD of a 6/8 wheel Drive Train for 6 months that revolves around the AM Sonic Shifter, which just went out of stock

My first question: What other shifting gearboxes are there out there (MUST be shifting)? I only know of the two Vex shifters, the WCP DS and the Vex Ball Shifter.
My second question: What are the advantages/disadvantages of each of the Vex shifters? What experiences have you had with them?

My third question: What is is the problem with making custom gearboxes? I know how to calculate center distance and all of the other design aspects, and I heard from one of the top engineers at NHMFL (Our local "National High Magnetic Field Laboratory") that the limiting factor is the machinist. Is this true?
What are the most crucial things to consider when designing and manufacturing a custom gearbox? What should we keep in mind when deciding to make custom gearboxes or order COTS ones? (I am planning to design the custom one with all the same gears/parts as a COTS one, just with a different plate layout.

Thanks so much in advance.