It's a bad benchmark because it's artificially clean. It's effectively a 2d dataset with no occlusions. So nearly everything you try on it will work, and many things you try on it won't scale to typical image problems. There are good 3d datasets with more realistic examples that are still fairly simplistic compared to the state of the art large datasets, but at least give you signal that your technique is robust to common problems in vision. MNIST is so simplistic that you encounter none of the typical problems in computer vision settings so it doesn't give you a good prediction of how good your technique is.

So MNIST for ML models is kind of like FizzBuzz for humans doing software development.