It is significant when it comes to color correction. You have less color information to work with, so you have less latitude in manipulating your image. Things can turn pretty blotchy pretty quickly if pushed too far and the results are less predictable. Encoding artifacts that aren't visible in the raw can become quite obvious. That sort of thing.
If you shoot a with your camera set up right so you have a good picture to start with your footage will hold up just fine in color correction using 4:2:0. I have been doing it for two years with a Sony EX-3 and for normal shooting there is no problem. You can push the picture quite far with ease. It does have a little more problem pulling a key than 4:2:2 but otherwise you will not have a problem. This is from my experienc not just theroy. There are also people who bash the Long GOP cameras as well but in my opinion they have never tried them. They work and edit just fine. Good luck.
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