YES (!) a custom frame is well worth it for us.
However we ride a tandem bike that has to fit 2 people.
Pilot (person up front is 5'7"), stoker (person in back) is 4' 10 3/4 inches 'tall'.
There are some tandems available that can be 'a make-do fit', however that's not what we need.
Have owned 4 custom tandems built by the following builders: Assenmacher (Michigan) in the mid-70/80s; put 64,000 miles on that one. Colin Laing (England and USA) in the '80/90s; put 56,000 miles on it. Co-Motion (in the 90s-2003) (Eugene, OR) put 57,000 miles on that. Now a carbon fiber Zona built in Arizona by Bob Davis and currently have 34,000+miles on the odometer.
All have been built to our specifications. All served us extremely well and all were well worth the $$$!
Know what you want, present the idea and drawings to the builder. Listen to builder's suggestions.
Pedal on!
Rudy and Kay/zonatandem