This will probably change (I hope!) with the adoption of the Language Server Protocol. I personally find Rstudio's suggestions to be a little odd (though I appreciate the fact that it actually works).
My two main gripes with the current ESS AC implementation is that (if I've understood it) it relies on an active R process (e.g. it won't suggest the name of a function you've just written unless an active REPL has evaluated it), and that it doesn't work at all with pipes, which means it fails for ggplot2 and dplyr style coding (which is really all I write).
I see what you mean. Yes, AC needs an active process. Personally, I use dabbrev-expand for things that don't autocomplete via the standard AC mechanism.
