While it’s quite true that Hitler served prison time for the Beer Hall Putsch, bear in mind the charges these fellows are looking at would land them in the slammer for at least 20 years. Murder is a damned serious charge, and add in extortion, bribery, blackmail, etc., and 20 years would likely be a minimum sentence at best.
If they’re convicted and found guilty - “if” being the operative word - these guys will probably be locked up for the rest of their lives.
Hitler's attempt to overthrow the government wasn't as serious a crime? Isn't what Hitler did treason or something?
European jail sentences are rarely as long as they would be in the US, though.
Arresting the Golden Dawn leaders does nothing about the social conditions that made Golden Dawn popular.