> Why is it "systemctl list-unit-files"? What the hell are unit files? How is this in any way logical?

I suggest you read systemd.unit(5), which begins as follows:

"A unit configuration file encodes information about a service, a socket, a device, a mount point, an automount point, a swap file or partition, a start-up target, a watched file system path, a timer controlled and supervised by systemd(1), a temporary system state snapshot, a resource management slice or a group of externally created processes."

> Why is this an argument and not a flag?

Multi-use programs with a good CLI accept the argument(s) determining what they do positionally rather than as flags. Flags should be used for options that somehow modify the behaviour of the command.

> Why is it that I can use a systemctl command, and it tells me to use the -l flag to view non-ellipsized output, but then when I use the flag, it gives me an arcane error that tells me nothing about what happened?

I don't know what you mean. For me, 'systemctl list-units -l' works fine.