Carbon tax would not be Pareto optimal. nobody is claiming it is are as far as I know and there is no reason to assume it is. Some people would lose big.
Large scale disruptions and frictions when economy adapts. Value of various investments will sink when taxation changes. People lose their jobs, some can't find new jobs with similar wage or with less wages. Even house prices will drop outside accessible public transport. Demand for electric cars would increase their price. For example tourism would suffer dramatically when people stop flying.
People in poor countries would suffer the most because their economies are very CO2 intensive.