If this is an Ubuntu installer, then you definitely want to install Ubuntu in a Virtual Machine. People who try to install another OS alongside macOS on the internal boot drive usually end up with problems especially when they try to remove the other OS and recover the storage space (or when attempting to resize partitions because they chose the wrong partition sizes).
If running Ubuntu in a Virtual Machine is not an option due to performance issues, then I suggest installing Ubuntu to an external USB3 drive so that it will minimize the interference with macOS on the internal drive. A USB3 drive will have the same speed as an internal SATA drive especially if the external drive has UASP support.
You have much more faith and trust in people & companies than I do.
Good luck.