Bytes: Custom ROMs have both pros and cons and it totally depends on the type of the custom ROM you’re using & device which you are using.
To me custom roms on OnePlus device is a total shame, the reason is OxygenOS is good, very good. It provides all the latest features & stock like experienc.
Why Custom ROMs Are Not Good.
Usually for installing custom ROMs unlocking your bootloader and installing/flashing a custom recovery is required, and as the time goes on and you enjoy your custom ROM, the custom recovery ( usually TWRP or CWM ) becomes your permanent recovery. No what’s wrong with the custom recovery? Well, te answe is simple, by installing a custom recovery you expose your data to everyone because when your smartphone is switched ON and you’ve a password/pin code set on your lock screen, then if someone steals your phone and connects it to his/her PC for accessing your files/internal memory, the Android OS will not allow him/her (Only if U have Android 4.0 or higher), because in the MTP PC connection, your internal memory is not showing unless you unlock your smartphone.
If you’re using a custom ROM and hence using custom recovery, then one can easily access your internal memory by booting into recovery mode and connecting your phone to PC, because recoveries don’t restrict users from transferring files between PC and Phone.
Besides that, you also don’t know what happens on the back end of a custom ROM, there is a possibility that the developer of the custom ROM is accessing your personal files/message etc.
Through there might be privacy issues in custom ROMS but some custom ROMS like LineageOS provide features which allows you to allow/deny apps/processes to use your data, contacts,messages,storage or anything in your phone.
The companies like Xiaomi, Huawei & other chinese manufacturers are providing a skin on top of their operating system, So installing a custom rom on their devices is a good thing & makes complete sence. You will get the latest OS & all Stock android features & perks.
And those talking about battery drains and lag/performance issues, I don’t think these are correct because the custom ROMs are kind of refined versions of the stock ROMs , and I’ve never used official/stock ROM in my phone because the custom ROMs provide more stability/customization and are fast. And all other issues mentioned in other answers could be fixed. Here is the list of best custom android roms.