I don't know much about the detailed history of the US and in particular the coal workers strikes of the 1920s, so I've no idea how accurate a portrayal of the Matewan Massacre the film was. However, it was very well acted, very moving and did carry across several strong messages.
The film clearly shows how workers could be exploited by their employers and the reactions that take place to this. For example, early on the film you saw the miners reactions directed to imported workers being brought in to replace those that joined the union rather than the employer that was treating them as a commodity and replacing them accordingly. This is exactly the same reaction as the modern "British jobs for British people" mentality. The jobs don't belong to the workers, they belong to the employer and the workers that fulfil those roles are essentially commodities owned by the employer. Modern workers may have a lot more freedom nowadays, but that basic fact is still true and as with all commodities the purchaser (employer) will seek to gain this at the lowest price and potentially replace them if a lower priced alternative comes to light.
The other key message the film put across was how violent uprisings weren't the answer to this. The main character, Joe Kenehan, when discussing not fighting in the war, highlights that is objection is that he couldn't take the life of another man who is just doing what his government tell him to. I've heard people put forward the view of soldiers as workers tasked with killing each other to promote the views of the governing class before, but never in a major film. It makes a change from the promoted idea that all soldiers clearly fought for a cause they strongly believed in.
However, even in a minor uprising like this one, where the miners were fighting for their rights, the film gives strong messages to show how this doesn't help. All the active fighting doesn't actually help the miners at any point in the film and it only ends up with casualities.
All in all, it's a good film, very well acted and very thought provoking.