pfr I think that for coding nvi is definitely too limited and doesn't keep up with most modern editors.
For scripting, editing config files and system maintenance, it is just as good as any other vi implementation.
I don't rely on syntax highlighting and that's IMHO the greatest difference from a user standpoint in terms of practicity, alongside the lack of plugins support. These are possibly the major showstoppers for anybody coming from (neo)vim.
Speaking for myself, as I wanted to learn vi 'for real' some years ago, the vi(1) man page appeared better organized (in a single file), more concise, and therefore more accessible for a beginner compared to the extensive vim documentation, so I picked nvi.
Now I manage all of my desktops, home server(s) and shell accounts using vi/ex and little bit of ed. I also use sh and editline in vi mode. I find myself comfortable with it and that's, at the end of the day, what matters most.
The good things about nvi are:
Extremely low memory footprint. vim (and especially vim+plugins or neovim) can be excruciatingly slow on hardware with very limited capabilities.
Included in base on all BSDs, so that's what you'll be likely using on a fresh install, or on a single-user session. Some times you may have screwed things enough for vim to complain about missing shared objects and/or for vim not to build.
The traditional and simplistic approach of nvi makes it adheres to original vi most. What is true in nvi tends to be true elsewhere too (with few exceptions). As I use elvis on Slackware and FreeDOS (again, I find vim to be slow on DOS), as well as original vi on Solaris, I won't have to learn anything new, nor adapt my workflow to a different editor.
portability and license.
Given these premises, I'll recognize none of these 'advantages' is of any practical utility in modern everyday computing.