That applies for ARM-based appliances which in most if not all cases run U-Boot... the problem is, in literally every appliance whose firmware I touched for one or the other reason the respective fork of U-Boot was extremely old, and the vendors did not publish the sources at all or lacking the configuration used.

In the mobile sector, most don't ship u-boot but some other bootloader, in this case mostly because u-boot doesn't support GUI operation/output which is vital for a phone.