The trouble is, the old hand built car line comes out a lot, but most of the troubles aren't due to workmanship in build or parts manufacture, they are inherent mechanical design issues.
How do they compare against niche vehicles like those I mention above? In fairness I don't know, but I suspect not that great.
Putting a large capacity (high torque spike and vibration) low volume bike engine through untested powertrain (cush drive and bevel box) puts the project on the back foot before it starts, compared to taking a well tested engine and gearbox from a manufacturers parts bin. It is, however, also what makes it interesting to the owners, and maybe that's the price to pay until development and design catch up.
