In practice, with limited computational resource you only can generate few first character of Bitcoin address (e.g. 1YourBrand...). The rest of characters still looks random and near impossible to remember without any mistake.
It is impossible to create a Vanity address with too many customized characters like more than 8. It is discussed in
Vanity address (Mastering Bitcoin book)6. The frequency of a vanity pattern (1KidsCharity) and average search time on a desktop PC
Length Pattern Frequency Average search time
9 1KidsChari 1 in 7 quadrillion 800 years
I'm not sufficiently knowledgeable on this, but impossible is probably a wrong word to describe it. There are vanity addresses with more than 8 customized characters. The book is apparently referring to what a normal desktop PC can do. For a GPU with a lot more computational power, however, I think beyond 8 characters is very possible.
Please take note that my weekly payment comes from a vanity address with not just 8 or 9 customized characters but 10, in all caps. And it's a legacy address at that. BestChange pays from 1BESTCHANGEuX2oUwodgvJqB52kTsrfXS9.