If a proposition can’t be stated in one simple sentence, then posters are probably the wrong medium for your message. There are exceptions though. If an ad can be placed in a spot where your audience has time to dwell – for example, across from a train platform or in a Tube train carriage – then longer copy can work.
The poster medium has changed so much with the advent of digital screens. You can now run basically a silent TV commercial on a poster site, but in most cases there really isn’t enough audience dwell time to do this.