The first thing that I noticed in the very beginning was the significantly worse directing, especially in fights. The pacing and cuts had only comedic function that disrupted the dynamic of action.
The dialogues were tacky, acting was often over the top, some of the plot was primitive, main characters lacked depth and communication, their actions were ill-considered and based on speculation or luck, which was probably the result of lazy writing. A lot of plot holes.

In the previous season there was interesting contrast between willful actions of Rainbow Taxi and law. I liked the tension of being targetted by a prosecutor and nuances where Kang Ha Na was on the fence of law and justice. She could have been a great addition to this season.
The rest of main charecters also had interesting depth before, because they were victims and criminals at the same time who faced the loss of loved ones and trauma, and were still unstable. They had a moral confict between their good intentions and the desire for revenge leading to committing serious crimes on a quite large scale. They have all lost duality aspect in the second season, but Jang Sung Chul lost the most. He was previously the most conflicted, most brutal and hid the most darkness within himself (as well as connections with criminals :')), but he became "family-friendly" too quickly.
There used to be a lot of darkness overall, both in criminals harming people seeking help and in the solutions to their problems by Rainbow Taxi, but in the second season everything was kind of bland.
The story with the villain just was there.

On the plus side some of the episodes were hilarious, we got Madam Lim comeback and directing got a lot better in the second half.