My biggest problem with the new UI is the font. Roboto, or whatever font they are using, is terrible to read a lot of small text. I wish the font could be set to Helvetica or disable web fonts for Gmail.
With the old UI I don’t have to scroll down to see the labels that I frequently go to. The new UI uses a heavier font that the labels list takes up more vertical space than they used to which is becoming a nag for me.