I had a gig for a contractor of some banks once. One of the banks was using 5.0.0.0/8 internally. I asked the guy if it doesn't lead to issues. All the time, he said. When they first came in, they just assigned each bank 1.0.0.0/8, 2.0.0.0/8, and so on. Number ten feels really lucky, he said.
This story is so silly it must be true. Thank you, made me laugh out loud.
As long as it's strictly separated from the internet it works. The Internetwork Packet Exchange, used for cellular roaming, is a parallel global internet with its own addresses and allocation policies.