I love how they start with “it sits between official and homebrew”. As though WOTC designers had anything to do with the gunslinger. It was a homebrew Matt Mercer made for Talisin before they had really even played 5e, since he wanted to port over everything from their home game which had been Pathfinder 1e. It has very little connection to the 5e design philosophy and Mercer has a history of being really bad at homebrewing crunch that can be used at other tables besides his own.

FWIW I ran a wild west campaign in 5e for a couple of years and my “guns” were just reflavoured crossbows with easier reloading. I had a player who went hard into it and played a sharpshooter using a rogue/fighter multiclass.