Usually when public transit networks start showing strain, it's during rush hours, when they're handling massive spikes in ridership. To give an example from Chicago: If you're trying to get on the train downtown at 6:00pm, you'll get on the next train, and probably get to sit in a seat. If you try to do it at 5:00pm, you stand a good chance of waiting 30 minutes for a train that you can even physically get on to. When you do, your ride will probably also take 50% longer than the 6pm ride would have. And maybe even 100% longer than a ride at 7pm would have.
You naturally get all the complaints from the rush hour riders. And yeah, if your concert were starting at 6 in the evening so that you'd have to brave rush hour traffic, then that would be a problem. But so would driving.
If, like most concerts, it was starting a bit later, you'd very likely have experienced smooth service.