Posters are fantastic but they cause the most problems
Because they are physical. TV & Radio are electronic or digital. Magazines we simply send the material to the printers & then it's their problem
When posters go wrong it's our problem. And 90% of the screw-ups I've known have involved Outdoor. And because there is a real cost involved in printing & distributing hundreds or even thousands of posters someone has to pay in the end.
Despite this I still recommend posters to clients. After all it's only media. Nobody ever died because of media mistakes. So just light up a cigar & relax when things go wrong... see the classic Hamlet campaign in the link.