OH, least favorite.

I was wondering why everyone is so sarcastic.

Well, buggy software (MIUI) and the surveillance. I mean, there's a higher chance MIUI bricks itself during an update than with any Alpha-release custom ROM (so I don't update... [But it's also due to Android often removing features from API and breaking niche apps]).
E.g.: Wallpaper bug (stretched or awfully upscaled after changing brightness), brightness slider not working, broken screen rotation, battery drain, swapped L/R microphone audio channels, FM radio cutting out, system crashes and waves of killed apps, camera app in landscape has some buttons off-screen, MIUI screenshot tool occasionally breaks and stock screenshot tool is used (with full-blast audio sound effect no matter current volume settings), notification access service needs to be manually reset for each app after the app is restarted, 120Hz gets laggy after toggling battery saver on and off and needs to be manually reset, dark mode is forced on all apps but the setting to change this sorts all apps in random order,... Also, the Poco X3 Pro motherboard has a tendency to die. I am on this phone's third motherboard currently.

Also... the volume buttons can get stuck. Xiaomi phones have issues with dust particles making their way into buttons and activating them randomly.