Hechsher Accepted by Locals

There is a certain hechsher (I will not name it here) found on lots of products that three leading kashrut organizations have explicitly told me is unreliable. But it is the only kashrut organization in a major city with an orthodox population of several thousand, and all the Orthodox Jews in that city say it’s acceptable and use those products in their homes and on their keilim. That organization supervises all the kosher restaurants in that city, and all the Orthodox Jews who live in and visit that city eat at those restaurants. How is that possible, and how do you deal with it if you visit that city and you want to dine out or get invited to someone for Shabbos?