In previous years I've gotten the coach from Cardiff. Not had any problems at all, always a slight fear I'm not going to get all luggage on but it's fine.
Missed out on coach tickets last year so did the train around the time of the strikes at 8am worried it would be awful and it was a breeze to get in no trouble at all, shuttles to the site were easy.
Getting back got up early to get the first train to Cardiff and again no trouble at all. Bit of waiting around but never encountered any long waits thankfully. Coach is more convenient but the train was cheaper surprisingly last year in the end.