Avantages
If you grew up in Japan and went through the rigid Japanese school system, you will find that Rakuten is probably one of the few best places you can work here. The English requirement, and some global exposure and reach will make this a very rewarding experience for you (compared to other Japanese companies). This can be seen by reading some of the other posts here. People are generally nice, and if you lived in Japan for many decades it will be a breath of fresh air among the traditional companies you will see here. For everyone else....
Inconvénients
For everyone else....... your patience and sanity will be tested. The endless paperwork, KPIs and meetings will quickly sap your morale and motivation. If you like (massive) unpaid overtime, "optional" meetings, more meetings, and below market salary - this is your place. The entire company culture revolves around micro-managing every task, meeting and email you do. Your powerpoint and excel skills will be put to the test. Family? Regular sleep? - for the weak-minded. People are routinely chastised and embarrassed over mailing-lists of over 20-200 people for petty things. While it has not happened to me (yet), it does not create an atmosphere of the place you want to stay long term. On the tech / IT side, if you want to be 8-10 years behind the curve, this is the place to be. Basically if you live here, find a smaller (or truly international) company. If HR is recruiting you from abroad..i would think it over...hard. HR will put on a good show under BAM!, reality hits you and its too late.