It's was good in starting but ruined in ending /rushed in ending

I really liked drama after started watching after 13-14ep drama started falling , many plots holes like chairman kang's son a character that was not even cast was showered with a lot of mentions, especially in the closing episodes. Even hinting at inmate Tak Kwangyeon taking the fall for him as the psychopath serial killer, how did even Chang ho found all gold and seo papers in last 3mins,
What was going on that secret laboratory? No one bothered to show what's going on

I really disappointed in drama after watching ending