Its a flaw in classic logic related, invented by humans, its a flaw that was missused by sophists many times. As well as sharp 1/0 without third state or analogous variability, ie you either breath in or breath out, nothing inbetween.
Reiser4 was clearly heavily sabotaged, as it would bring huge advantage to linux. Hans has made two mistakes - married an opportunistic woman and lost the situation out of his control. I heard the system is usable, but it is lobbied by RedHat for whatever reason, and only russian developer,himself not very much interested, patches it for every new kernel. The only problems I heard were loosing data integrity(? don't know if its resolved) and mounting the system with image of this or other reiser4 system residing on that disk in form of file. The system itself should be (much) more advanced than btrfs itself.