In my opinion, a tool doesn't need to come with every functionality that exists and are cool if it integrates well with others solutions. And RStudio IDE integrates well with others for sure.
In the case of diff tool, Git integrates well with those, and RStudio integrates with GIT so you could leverage that using git command to launch a diff between 2 files. Also, RStudio IDE includes a diff viewer between files when you commit. I did not try to activate it for any comparison... do not know if it is possible...
Moreover, as RStudio has a great terminal now, it is also easy to call for your difftool with a command line. If useful, you can make a small R function that take 2 file paths and launch the third party app with an addin for better user experience.
Finally, you also have an R
for this that could help you but I did not try it myself.
It uses the javascript library codediff.js. Contrary to other difftools, you does not need to install one. It seems included inside the package. You can give it a try and tell us if it is interesting.