I have read a lot of Burroughs, and this one is rather typical of his stories. The idea of the civilized man having to survive in a savage part of the world is an all too common plot line for him.
The initial idea of European civilization being severely damaged by World War I, while an isolationist North and South America growing to the height of civilization and peace was a brilliant one. The idea of someone from the Americas entering the unknown European realm is a fascinating plot idea. Unfortunately, the book was just not long enough to really develop the story.
Even if World War I had gone as badly as the story indicates, I do not believe that European civilization would have been so totally obliterated that no trace of it would remain. There should have been ruins, at least. It seems more as likely that some sort of Medieval-type society would have resulted, especially since that is so much a part of the history of that part of the world.
Further, when the Roman Empire fell, some learning was preserve in monastaries. It seems to me that something similar would have occurred if World War I had destroyed European civilization. I kept expecting the main characters to come across something of this nature, but they never did.
Third, I find it unlikely that the animals that Burroughs describes as thriving in England would be able to do so, unless the entire climate of the planet had changed, and there is no indication in the novel that this has occurred. Lions and elephants may be able to live in zoos but if turned loose with a few British winters (from what I've read of the British climate), they would certainly not become more populous than humans.
Finally, I felt that the end of the story was rushed. With the material that he had, Burroughs could have stretched this story out to a multi-hundred page novel. As it is, the edition that I read was under 100 pgs.
In all, Burroughs started with a great idea, but it just needed a lot of work.