The main drawback is that it's really too heavy for what it does.
I don't understand the complains about the new design being image-heavy, or in general so much different from the old one. Once you set it (once) in "classic view", it's basically the same.
If I load an old/new reddit tab and go to 'about:performance', here's the comparison:
Old - Low impact - 22 MB memory usage - 4s to fully load on gigabit
New - Med impact - 70 MB memory usage - 10s to fully load on gigabit
But a lot of other critics feel like the person haven't tried setting the view to "classic" or "compact".
There is an add-on called "Reddit comment collapser" [0] that adds this to old reddit.
[0]: https://github.com/tom-james-watson/reddit-comment-collapser