There are difference in how the different branches operate, though (every federate state has also their own agency) and some seem to do good work, but others (thuringa) seem to be made up of Nazis themself, who rather support the neonazi organisations, they are supposed to be fighting against.
Frankly, I don't see a strong argument why they should exist in the first place and why their work cannot be done by ordinary (professional) police.
So yes, neonazis (and left wing extremists and islamists) operate in secrecy. But so do common criminals.
Edit: And the actual dangerous nazi terrorism cell that operated for years was indeed caught not by the specialised secret agency Verfassungsschutz, but by ordinary police. (With strong hints, that they were protected by some parts of the Verfassungsschutz)
https://en.m.wikipedia.org/wiki/National_Socialist_Undergrou...