Pros
UMC is a traditional company with a lot of job security. They try very hard to not have to lay off any employees (it's seen as a place where you can work until retirement). Also, given the size of its business (large for a Taiwanese company), it affords a lot of different experiences to its employees. It is seen as prestigious (in Taiwan) and since it has fabs in many different countries, you will have travel experiences as well. Good vacation package and benefits package. Also, there is good work life balance. Finally, there is a good mix of younger and older employees.
Kontras
There is a lot of internal politics and bureaucracy. This sometimes makes it quite hard to get the information needed to complete a project or to implement anything cross functional. Also, like many other big companies, promotions can sometimes be more about who know rather than what you know. There is a heavy emphasis on seniority within the company and there are cases where it seems that if you've been around long enough, you will be given management opportunities over someone who is more capable. This creates dissatisfaction among the most valuable junior employees and contributes to higher turnover and attrition.