Yeo Sangjae's fatal weakness is he doesn't make contingencies. He becomes satisfied with whatever plan he comes up with on the spot. Incessantly arrogant and complacent.
This is why he's far below his older brother. He wants to rise above his brother and trample on him but is unable to recognize his own flaws and thus can't correct them. He will always be inferior in both his family and to the bloodline that created the company he's in.