1. There is always a problem if a Jew is involved in case due to a halacha that one needs to use a Beis Din. That’s the last thing most Jews want to do. One can avoid the matter by specializing an area where that isn’t an issue (administrative law including tax law, since the non-Jewish “king” is a party so Beis Din isn’t involved).
2. Convicting the innocent and aquitting the guilty is a moral issue, but as long as you follow the rules of legal ethics, probably okay.
3. There are halachos about oppressing the poor, but as long as you don’t want get rich, that won’t be a problem.