Rugby league crowds for many years have had unpleasant elements.
I think crowd behaviour has moved along with shifts of the type of people who go to the games.
When I started watching in the late 80s and early 90s, rugby league crowds could be quite intimidating. The crowds were mostly male and there were a lot of young lads who were up for a fight. It was a bit like football with semi-organised hooliganism, and when we were playing teams like Widnes, Saints or Hull there was usually something kicking off at the train station or outside certain pubs.
Then in the early 2000s there was a big marketing push to make rugby league a family sport, which worked to a certain extent. You had a lot more families and a lot more female fans which diluted the tanked up guys. There were still often stories of crowd trouble on forums like this but one feature of the 2000s from my memory was I would often come back from a game and read 13 page threads of people arguing about crowd trouble when I'd never actually seen any trouble myself. It was probably isolated incidents.
From the 2010s onwards I think the demographics shifted again. I don't have any statistics to back this up but just from what I observed, RL hasn't had quite the same happy family feel. You do find some "hangers on" who turn up especially at the big games who don't appear to be very clued up about the team but who see it as an excuse for a booze up and to behave like a total idiot. It's different from the angry hardcore which I remember from the 80s and 90s.
One thing I would say though for people who are worried about going to Wembley is Wembley doesn't seem to be nearly as bad as the Grand Final at Old Trafford. London is a long way to travel for a casual fan and the kick off is in the afternoon. Very different from Manchester with an evening kick off where anybody who supports a RL team other than Catalans can hop on a train in the morning, be in Manchester before lunchtime and spend 5-6 hours getting hammered.