The ".ignore" name was actually suggested by the author of ag (whereas the author of rg thought it was too generic): https://news.ycombinator.com/item?id=12568245
It's nice and everything, but I remember being happy with the tools before (I think i moved from grep to ack, then jumped due to perf to ag and for unremembered reasons to pt.)
It took me a while, but I remembered I ran into an issue with pt incorrectly guessing the encoding of some files[0].
I can't remember whether rg suffered from the same issue or not, but I do know after switching to rg everything was plain sailing and I've been happy with it since.
[0] https://github.com/monochromegane/the_platinum_searcher/issu...