Major one for me is their software. No matter the cost, I don't think an Operating System should be crafted in a way that sneakily consumes user data and delivers ads to the user by default. Not on the lockscreen, not in the notification tray, not in the system apps like the file browser. Also, transmitting requests in incognito mode to their servers by default, can you really trust them?
You should just compare xiaomi phones based on price and ignore the model numbers. Depending on your location, there may be deals for certain poco or redmi models that make it easier to choose.
Well given Xiaomi's history of rebranding it's older series phone and launching them as new.
The only reason you should buy Xiaomi phones is to slap them with Custom ROM as soon as you get them. But given their recent track record of delayed kernel release, it's not looking too bright for them.