Ironically, the biggest problem with the Flash Gordon rpg is Flash himself. No matter where you go on Mongo, he's been there first. Now, to be fair, the only reason any of these locations exist in the first place is because they were the backdrop to his various adventures, but beginning each section with a recap of Flash's deeds lends the setting a certain degree of repetitiveness.
Let's just say that Flash Gordon's arcs tended to follow a pattern - the local beautiful queen would instantly fall in love with Flash and he would proceed to save her white civilization from an ambitious usurper who was also a spurned suitor, and at some point while this was happening, there'd be a monster attack and a run-in with the racially-caricatured natives who live in the wilderness between cities, and the end result is that the kingdom winds up becoming an ally in the fight against Ming (or, at least, commits to remaining neutral), often after an arranged marriage between the beautiful queen and one of honorable manly men that had sided with Flash in the past. Sometimes the queen is a princess, but the important thing is that Ming never comes out of these situations stronger than before.
It has the cumulative effect of making the rpg's timeline a little vague. Your PCs are some of the less prominent rebels against Ming, and so they're always thwarting some plot of his or another, but if you take the book as a guide to the status quo, then his empire has already been reduced to not much more than Mingo City and the Land of the Lion Men. You're pretty much treading water until the final battle to depose him once and for all.
Which is fine, I guess. I'm not sure I'd have made the choice to set the game in the nebulous time period between the comic's last arc and its second-to-last arc, but I can see why people buying the Flash Gordon rpg might want to roleplay in a world where Flash Gordon is out there doing his thing. It's the same issue I had with Pendragon - I really like the source material, but I have the perverse desire to replace the original characters, or, at least, participate in their iconic adventures.
I've never seen a licensed rpg that actually pulls the trigger and just assumes that you're going to play the canon characters. Probably because "improvised fanfiction with limited OCs" is one of the few ways to make roleplaying sound even dorkier than it already is. Also, with something like Flash Gordon, where there is a single main character and his supporting cast, it might be a problem to balance the spotlight.
Setting aside my nebulous internal struggle between a desire to recreate my favorite fictional moments and the need for an open-ended pastiche that lets me do things like my favorite fictional moments, how is Kingdoms of Mongo as a book? Well, the decision to leave out Mingo City was pretty much unforgiveable, however as a setting it's kind of great.
How to put it? Imagine you have a brother or sister who is just a little too young to GM, but you've been letting them sit in on your rpg sessions for a few months and they've been bugging you to let them try. So you do, and they show up with a spiral notebook full of campaign notes and it turns out to be a catalogue of everything they've read or watched for the last two years and the resulting game is basic as hell, but also kind of awesome.
That's Kingdoms of Mongo. Arboria is a forest kingdom full of Robin Hood cosplayers. Coralia is basically Atlantis with the serial numbers filed off. Valkr is a jungle kingdom ruled by a caste of Amazonian warriors who ride around on giant wolves (oh, okay, "Wolfins"). Radiuma is a land where the early enthusiasm about atomic power was never crushed by the reality of cancer (The radium miners have been exposed to a lifetime of radiation? Awesome! Now they have glow-in-the-dark skeletons!). If there was something people in the 1930s and 40s thought was cool as shit, it's somewhere on Mongo. The whole thing is a love-letter to pop-culture omnivory.
For an actual roleplaying campaign, I'd probably advise you to use one of the myriad of later rpgs that is self-consciously trying to tap into the same feeling. A lot of retro-throwback sci-fi has Flash Gordon deep in its DNA, but something specifically designed to be an rpg isn't going to have the same canon baggage and something contemporary is going to deal more sensitively with the genre's more problematic aspects (ironically, Mongo feels almost back-handed feminist because so many of the kingdoms have female rulers for Dale Arden to be jealous of, but I have a feeling that their depictions here have been heavily curated). You could even use Ukss!
Still, when all is said and done, I absolutely love owning this book. It feels like a bridge into the past, a connection to some immortal tradition of dorkery, and if it's a little simple, it also has an appealingly primordial lack of pretension.
Ukss Contribution: Aeroceti - sky whales. They're whales, but they're in the sky. Like I said, this book will get you back to basics in a big way.