What was the difference between Jackie Jnr sticking up the card game and Tony and Jackie sticking up the card game all those years ago? Was it just all about not having the connections, was it that no-one got killed when Tony did it, or what?

The primary difference is that no one was shot at, wounded or killed in Tony and Jackie Sr's move on Feech's card game, the only damage was monetary and pride - things that were then negotiated on and politicized.
Jackie Jr and gang, along with lacking respect & connections wounded Furio and shot at Chris, Albert Barese and Eugene - all Made men, something even a legitimate or recognized crew wouldn't be forgiven.