The main problem is that it is entirely and woefully inaccurate, lionizing Cromwell as a hero. It tries to do too much and condense a long, complicated period into an overly simplistic story. Harris was intense but pretty uninspiring as Cromwell. There's a lot of shouting. For me the films greatest quality is it's visuals, although the plot is severely lacking it does a great job bringing the period to life.
Very aged. Richard Harris is good, but Alec Guinness's Charles 1st (variable Scotch accent & occasional stammer notwithstanding) is wooden. The big battle scenes are interesting ... they would be very different nowadays. The indoor scenes and minor characters lack all the life and philosophical fervour we expect from Game of Thrones and Wolf Hall. Not good enough to be a classic.