I would say that if a frame cracks due to a design flaw/too much stress in a particular area it is probably not fixable.
My cross frame cracked at a point where the original design appeared to be a weak point. Rob Hayles – yes that Rob Hayles – at Recarb repaired it and strengthened the area in the process, so that’s not necessarily true. I guess it would depend on the exact design, but i wouldn’t assume that some design flaws aren’t fixable.
That would have been some resin and fibres plus expertise. By contrast a design fault on a titanium frame resulted in a trip across the Atlantic and the headtube and downtube being replaced. The carbon repair was about as local as it gets.