[bigfatkitten]

If you’re dual stack, your OS will prefer IPv4 to ULA and ULA won’t be used at all, and so the extra config overhead of deploying ULA is pointless.

[Dagger2]

It'll prefer ULAs when connecting to hosts without A records. Programs will use ULAs if you connect to an IP literal, or if connecting to the A records fails. Also, Linux/glibc will prefer ULAs if you have a ULA assigned to the machine, and so will anything using the update to RFC 6724. So "ULA won't be used at all" is definitely not correct.