Of the two that have their own Wikipedia pages linked from there (Tutonish and Folkspraak) I can just about understand almost all of the examples given, though they are very short and selection bias may be at play. Also I speak English, German, and Norwegian, so I guess I have an advantage over speakers of only one Germanic language.
Especially when learning Norwegian I noticed that many words are obviously cognages with either German or English, but not both. For this reason I'm skeptical about the possibility of one vocabulary that is understandable to speakers from all branches. I don't know any Slavic language but know French and some Italian, and the Pan-Romance languages listed elsewhere in this thread seem much more readable to me than these Pan-Germanic ones because their vocabularies are more similar, I think.