> I didn't switch from ag, [...] rg using implicit '.ignore' file (a super-generic name instead of a proper tool-specific config)

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

Totally agree with the author of rg here. Config names should be unambiguous. Anyway, must have been something else, then. As I've said, I cannot remember what was the specific problem, only that it wasn't quite compatible with the workflow I was used to, and now it'd take another full-in attempt to switch to figure out what was so annoying to me back then.