Devtools is generally stable, most of the active development has moved to sub-packages like remotes and usethis, and relatively little code lives in devtools itself.
I don't know of any major outstanding issues in devtools itself.
The package development cheatsheet likely needs a major overhaul to account for much more functionality in usethis and the current state of devtools. r-lib#2107
Should devtools be converted to use pak for installation, or should the installation commands be deprecated in devtools and users suggested to use pak directly?
devtools has some vintage tests around .Rnw vignettes.
It's not clear if it makes sense to keep these, but I (jennybc) am not rushing to remove these tests.
Since I inherited the maintainership of devtools, I have not pursued why 2 of these tests fail locally (they are skipped on GHA).
However, today, I did sort it out and want to record what I did.
In case it comes up in the future, this caused the necessary LaTeX package to be installed:
tinytex::parse_install(
text = "! LaTeX Error: File `grfext.sty' not found."
)