You're absolutely correct! Only several 3, 4, 5, and 6 letter swear words/phrases and just a couple of combinations of double 3 letter swear words and only a few "U" swear words can be spelled out.
To be honest though we have been running this scheme for a few years (around 300,000 orders since we moved to using it) and although a few have appeared we have never had any comments. I suppose it is quite childish I guess to "spot" swear words in order numbers 😉 - and especially with the hypens they become less obvious.
I am the business owner so no one is going to shout at me either!