The vendors differ in their attitude to unlocking the bootloader. Some require codes for unlocking, some not. Some require proprietary, Windows-only software, other not.
Which of the vendors are friendly to people unlocking the devices and which not? Which of them require obtaining tokens, which require proprietary and not-cross-platform software? Which treat unlocking the bootloader as voiding the warranty?
This is only devices which can run Android.
(I started the community wiki)