At my previous company I designed small boats and spent considerable time in coastal Maine, the center of custom boat building in the US. Being an avid sailor I was always interested in seeing custom boats and chatting with their owners and builders. While these boats were beautifully designed and finished, I was always surprised by all the technical issues their owners encountered. Despite a cost of half a million dollars or more, there always seemed to be mechanical or electrical gremlins. The best production boats, particularly from the established Swedish yards, had relatively few problems and the construction quality, though not the finish quality, was often equal or better than custom builts.
I've found a similar situation with custom bike frames. Of my four custom frames, three had problems that I probably wouldn't have seen on a better production frame. None were major problems, just little annoyances, but still... Chatting with other custom frame owners, I found that many have had similar experiences. That is one reason that the (now discontinued) Velo Orange custom frames were, in fact, semi-custom rather than fully custom. I knew that making each frame completely different would lead to unforeseen problems. Instead, we chose to make three stock designs and offered only custom sizing, paint, and braze-ons.
Why is it that custom frames, boats, and probably other bespoke things, suffer from issues rarely seen on top end production products? I think the answer is simply experience, both the builders' experience and the fact that there are no prototypes on which to work out the bugs. The custom boat builders have built a few dozen boats at most, each one taking upwards of a year to complete. The custom frame builder needs a week or two to make a frame. The Swedish boat factory builds a hundred boats a year and the top-end bike factory makes thousands of frames a year. They've made most of their mistakes long ago. The guys at the factories who weld or braze are also likely have much more experience than the custom builder. And, despite what you may think, they often take just as much pride in their work. In addition they have the very best mitering, alignment, and other machines. Now please remember that I am writing about the very best bike factories, not the ones that make mass-market bikes.
In my experience at VO, we have had far fewer problems with production frames than customs. That is not to say that our production frames never have problems. They occasionally do. I once sent back 30% of a frame production run (to a factory we no longer use, but that still makes frames for one of our competitors). And some things can go wrong with any frame, like the fender eyelet that recently broke on one of ours. That dropout is used on both production frames and customs; short of x-raying it there was no way to have seen the flaw. We sent out a new fork ASAP, while the custom builder would have to make one and have it painted to match.
Does this mean that factory frames are better than customs? Of course not. The best custom builders still make the best bikes in the world and have waiting lists many years long to prove it. They are the exceptional craftsmen with a near pathological obsession for detail and decades of experience. Then there are lots of just average custom frame builders. We tend to romanticize the artisanal object, but just as there are good and bad factories, there is a range of skill among frame builders, boat builders, wine makers...
So, should you buy a custom frame? If you are of average size and weight I would argue that you don't need one; there are enough good production frames to choose from. And, based on my experience, you may well have less trouble with a production frame. A great production frame is usually superior to an average custom. Of course, someone who is extremely tall, short, heavy, oddly proportioned, etc, may get more benefit from a custom.
But, we want what we want, not what we need. Experienced cyclists with refined tastes will derive great pleasure from a custom bike and that may well be what matters most. They should be prepared to look for the exceptional builder, have a long wait, and expect a big bill. My advice is to take your time picking a custom builder. Talk to the builder's clients while remembering that few will admit to spending a fortune on a sub-par frame.
Anyway, that's just my experience with custom and production frames. The point is that the idea that you won't be happy until you own a custom is just as ludicrous as the idea that no one should have one. Anyone who tells you that you will get better quality, performance, or durability, in a custom probably just has a frame to sell. What you will get, if you choose wisely, is exactly what you want in geometry, stiffness, finish, and those little details that make the frame yours. Not a bad deal really. My two favorite frames are my semi-custom VO pass hunter and my semi-production Ebisu all-around frame. If I damaged either of those frames I'd build up a Polyvalent for myself (I get a discount on those).
What do you think about custom bike frames, or boats?