In most Japanese classical budo, the vast majority of training is done with a partner, and in the modern budo there is generally some form of randori . Training with another person with whom you have to demonstrate the effectiveness of your technique forces you to be honest aobut your technique. Unless your training partner takes a dive for you, you have to be honest about how well you are doing a technique and what needs work.
When using a live blade in iaido, it's difficult to find sensible people who are willing to act as training partners. You have to train solo. Solo training means you don't have independent input on your technique. The most difficult thing about iaido training is that you have to look at yourself and decide if you're doing things correctly. I don't know about anyone else, but I have a nearly limitless ability to convince myself my technique is great. Without the check of a partner, I can tell myself all sorts of stories about how fine what I'm doing is. Good iaido demands that you look at yourself without flinching. I have to think about what I'm doing very carefully so I can't let little self-deceits slip in.
I have led myself down some disasterous dead ends, even when people whose opinion I should know enough to listen too have told me how wrong I was. Even looking at myself on video didn't help much. My river of training started flowing backwards for a couple of years while I listened to the lovely lies I was telling myself. It was only when I started to look at myself without consideration of how good or bad I was did I begin to fix things. I had to stop telling myself that this or that works better for me, and only ask "How close is what I am doing to ideal iaido?" When I started doing that and looking at my iai without flinching I started making progress again.
Looking at myself like this has been really hard on my ego. As much as I try to convince myself critizism from others isn't quite right, it really hits hard when it is coming from own mouth.