My sentence is "The system was so badly designed that it has now become rickety. Even a minor change could cause the entire system to collapse". Rickety just doesn't seem to fit well.
You can use "(extremely) unstable" in place of rickety. When you describe systems and computer-related hardwares and softwares which are not stable in operation, you use "unstable" which is defined in Wiktionary:
(1) Having a strong tendency to change. (2) Fluctuating; not constant. (3) Fickle. (4) Unpredictable.
You could use unstable or unsound. Fragile would also work. Rickety seems to fit better when referring to furniture or a stair-case.