1. Any examples of cases?
Typical cases change according to the office, I would recommend you specify at least the country interesting for you. Having said that, it is risky to prepare only on “common cases” as there are no guarantees you may get a "common" one
2. How much difficult?
Not sure how a scale of difficulty may help;) If your question is if it is easier to get into RB than MBB: not necessarily. Companies hire based on demand and supply. There are fewer people applying at RB, but given RB is smaller there may be fewer slots as well
3. Are the cases depending on the practice of the interviewer?
Possibly, but not necessarily.
For additional information please feel free to PM me, I have 40+ real questions asked at RB worldwide.
Hope this helps,