Protonerd:
Does this bug affect all boards or only certain "technology corners"?
It affects any board that uses the ATmegaBOOT_168_atmega328.hex bootloader, Duemilanove, Diecimila, Nano, Arduino Pro or Pro Mini (5V, 16 MHz) w/ ATmega328 and possibly some of the other boards that don't use optiboot and don't have WATCHDOG_MODS defined in the makefile, I haven't looked into the bootloader source for those. The issue: ProMini: Reboot loop when using watchdog · Issue #150 · arduino/ArduinoCore-avr · GitHub is that the watchdog is not disabled by the bootloader so if it was enabled in the sketch and times out during the bootloader then it can go into an endless reset loop.
You should be able to just select the Uno from Tools > Board menu before you burn the bootloader and then the users of the DIYino will need to remember to use it as an Uno.
I use a lot of the Pro Minis which come with the ATmegaBOOT_168_atmega328.hex bootloader. I always burn the Uno bootloader on them right away and use them as an Uno instead of Pro Mini.
Protonerd:
does the uno also have an Atmega328P? Because I once tried to bootload a non-P as nano instead of mini and it failed...
Yes, Uno is ATmega328P