Pros
Pros and cons highly depend on the department and team you end up in, so I will write about my personal experience. But keep in mind that there are a lot of people who have exactly opposite situation. Rakuten is almost the only Japanese company that won't require Japanese language and you can apply without any knowledge of Japanese. In my team we always use English in all meetings, even though about 80% of members are Japanese. Work/life balance, overtime, etc. also depends on your team. I never did overtime in Rakuten, but I know people who stay until 10-11pm almost every day. My managers always treat me very well and with respect. I joined as a new grad, but my team treated me like a high-skilled mid-career employee. Me and other new comers were given tasks that require very high level of responsibility. However, for other departments it may be exactly opposite. Rakuten is a huge company with different cultures. Everything depends on on your department, your managers and teammates. But don't forget that it is still a Japanese company. If you apply from overseas and never lived in Japan, you will have a lot of challenges. But hey, it's Japan. Don't expect western-style management, culture and traditions. I see a lot of people complain about their work in Rakuten, but compared to other Japanese companies, in my opinion it is not so bad.
Cons
The worst case is your team doesnt use English at all, you are not given any tasks, managers treat you like a little kid or without any respect, you do a lot of overtime, etc. Very bad OJT training that required you to do meaningless meetings, presentations and tasks. For new grads - very bad HR training that requires you to do even more meaningless things (for example engineers have to sell Rakuten cards). For new grads - you don't know department that you end up in (it's almost random). Low salary. Too many meaningless meetings, Japanese business style. No innovations, too much copying from competitors.