(As to the rest: I use a pretty minimal set of plugins and I use the built in nvim C-o/C-p or C-x C-o/p "dumb" autocomplete. At least I think it's built in...)
To answer the question: it's a feeling, like lots of things in software development. I tried "no syntax highlighting", found that I liked it, and I no longer use syntax highlighting. To say "specifically" how it's "better"... I'm not even saying it's better. "I like no-syntax-highlighting" is the statement I'm making (which, when it comes to syntax highlighting, is a statement a lot of people have issues with). So, from my personal experience, I take issue with the statement that no-syntax-highlighting is making things "difficult for the sake of it".
Try this out for analogy: I ate Red Baron pizzas every Friday night for 15 years, then I heard about homemade pizza 10 years ago. I tried making homemade pizza. It was good! ("I tried it and liked it") Now I only eat homemade pizza on Fridays. How is homemade pizza specifically better? It's better because I like it more. That's all there is to it. It's a preference.
(For the analogy to work, you have to like or at some point have liked Red Baron frozen pizzas. I happen to like them... the analogy is flawed though, I admit!)
(Let me preempt criticism that I'm comparing Red Baron frozen pizzas to syntax highlighting. I am not. It's only about the preference, not the object of the preference.)
I have my editor configured with zero highlighting for keywords and syntactic elements. Admittedly, I have compilation/lint/syntax/type check errors set to invert the erroneous block, black background white text.
Syntax and keyword highlighting is just noise given I’ve been trained by decades of colourblind unfriendly interfaces
And I'm using more my mental idea of the code and direct line:column navigation. My context is often less than 10 lines of code.
I'm not saying you need every plugin ever, but autocomplete?
Maybe you haven't tried "advanced" editors in a while, and it was a lot worse last time you tried?
The trick is to avoid idle browsing of the code. Be intentional about what you need to do.
I use tools like grep/ripgrep to get a more focused of the code. Then with the line number, I can jump directly to where I need to be. Same with tools like linters and compilers.
In the same file, I often use search instead of line/character movement. You search for a symbols and you just cycle through their location.
I don't think it leads to a better code. But badly organized code will make this harder, so you tend to think about organization.
Re: syntax highlighting, I don’t know how people can work with their harlequin-on-LSD themes that are a constant distraction with no semantic benefit. I have gravitated around mostly mono or duochromatic themes, while 99% are a vomit of colours. I still don’t get why variables or function names need a distinct colour.
Can’t really justify it at work though, projects are too big to and gnarly keep in my head.