Using a large row of textless icons can be OK if the meaning of the icons is well understood. If all you have is
"house" - "person" - "gear" - "bell" and the context is a social media platform, you probably can expect a user to figure out that this is
home - profile - settings - notifications.
Once you go anywhere beyond that, icon-only becomes very tough. Take these guys for example:
Even if I was to tell you that the things in the light toolbar mean
Search, Home, Conversions, Behavior, Acquisition, Audience, Realtime, Customization, you'd be hard-pressed making the right connection from each meaning to each icon. (Correct answer in case you're interested: 1,2,8,7,6,5,4,3)
The ones in the dark toolbar are so cryptic and similar to each other that it becomes near-impossible to tell which is which - hence why they're always accompanied by text. The site in question this dark sidebar is taken from is YouTube, btw.
Removing labels from a sidebar and making them icon-only makes things harder to scan.
Taking back a step from that:
it takes longer for users who are accustomed to your platform to identify the button and navigate to pages by an overload of information.
A navigation sidebar containing some icons and 1-2 words per item is very unlikely to get overwhelming. Especially if you give the sidebar some internal structure (like YouTube does, for example).
Documentation pages have a tendency to have overloaded sidebars, but even then, typically that's a result of the nesting depth becoming too wild, not necessarily a result of there being too many words.
Given that we have most, if not all, large user-facing sites use sidebars with text, I'm inclined to say that the benefit of showing text significantly outweighs the benefit of having a smaller sidebar, on desktop sites anyway.