The problem with a custom frame IS that your requirements could change, thusly making the rider need adjustments. This is why I say to never get a bike with a rider weight limit, you never no what could happen to make the bike not right anymore.
Unless you have out-of-normal sizing issues, the custom bike is probably not for you, unless there is a material/construction method/paintjob you have a lust for. A lugged steel bike is nearly available only by a custom manufacturer, and good lugs are getting harder to come by, as well as the tubing to make a nice, lugged steel bike. I will say that if all steel lug production were to cease, there would still be quite a few builders who will have enough lugs for their lifetime, if not part of their childrens’ careers.
Joe Average age grouper could end up with a bike underbuilt if he has a particular winter where he is recovering from bad injury, with needing a different headtube or top tube measurement until he recovers, and sometimes, that could be a case where Joe Average age grouper would NEVER be able to ride the bike again.
If custom IS your undying wish, then get a bike made for sport riding, unless you are a guy who can be consistently be at top of the pack finishes. This could end up being a bike with stock geometry, unless you are hard-to-fit. It has been mentioned before, but pro riders will generally have a few customs built per year, especially if they switch sponsoring manufacturers. And let’s face it- the better bike brands that cost more will have more sizes, which would then fit probably nine out of ten consumers with quite a bit of overlap.
My experience with a custom is this: long wait, a bike not suited to my style or weight, and a bike that was sold the following season. That being said, I probably had just used the wrong manufacturer, judging by the number of older Mandarics, Yamaguchis, and Landsharks I see running around here on a regular basis. I am not soured by customs, as I will probably get a custom frame as my next one.