migration assistant with ethernet crossover cable

Connecting both Ethernet cables to your Router may be more reliable. You should also make Ethernet the TOPMOST interface, so it will be used preferentially when connected.

When you just connect a crossover cable, there is no central authority to provide IP addresses, but each device HAS a self-assigned IP address, so it should be able to work. But your Mac may decide to use Wi-fi anyway.

What does NOT work is just using a USB-C cable, because USB is considered a local interface, and you need a Network Interface.