1) There are few PhD positions payed directly from the University. For the rest people it depends on the funding/research programs of the supervisor (personally my salary was payed initially from a European project and then a national project that my supervisor was participating). So some attention in this matter, to be sure that you will have funding throughout your PhD.
2) The German language issue: The group I am working is quite international, so as a foreign student I didn't have many issues regarding the language. However, sometimes it is quite hard to cope with, if you don't speak German well. For example the working contract is in German and some forms are issued only in German. Additionally, there are very few available courses in English; the vast majority is in German.
3) Working hours: The working hours if you need to finish your PhD within the minimum of 3 years can be quite long. Technically PhD students are payed for 30 hours weekly (this is the official full-time contract for PhD students), but on average work at least 40 hours/week. Personally, I work around 50 h/week. Of course this depends on how long you will have funding.
4) Some discrimination among individuals exists (personal or national criteria), but this is totally in respect to the individual supervisor/PI.
5) Supervision in my group is not sufficient. You need to struggle a lot alone, if you want to progress.
