I just finished reading Worlds Without Number. It is not a game I would ever want to run. I would play, if somebody offered to run it. Tracking Effort seems like it would be a total PITA. The GM advice, campaign creation chapters, faction handling and everything else are excellent. You need to know how much to pay the spymaster that trained the guerilla fighters embedded in a city, it's there. And the rules to track the effectiveness of said guerillas. The rules for how failed magical research twists the resulting spell/work are really cool. There is so much in there, which is awesome, and the problem.
I finished reading The Atlas of the Latter Earth last night, to time to expound on WWN a bit more. The Atlas is impressive - a mostly cohesive 4000 years of history of all of two continents. There's a country or region for almost any non-scifi adventure you can think of.
Consistency can get a bit wonky when you cross borders - the Sarth won't sacrifice an adventurer because the they're more trouble than they're worth compared to slaves, volunteers or the generally devout populace, yet all the neighbors have border forts to fight off raids? But it mostly fits together.
The bestiary is meh. Latter Earth flavours of your standards, but it's a nice addition because the bestiary in the main book is pretty minimal. The atlas has partial classes for low and no-magic games; foci for wuxia-style, various divine gifts, arcane secrets and demi/non-human characters.
The chapter on ships and naval combat is extremely comprehensive. I could easily see pulling that into other games.
The repeating "don't prep it if you don't need it" across both books is probably overdone (me remembering my prep for Into the Cess & Citadel and I was still blind-sided by you guys releasing all the animals in the zoo) but good advice for such a huge sandbox.
From my admittedly very limited perspective, the effort, shock and system-strain mechanics are fairly original. I've seen this type of elf before in fiction, but not rpgs and the dwarves seem completely original (there are optional rules to skin them both as more conventional versions). Other than ship mechanics, my favorite part of both books are the tags. Tags are basically oracle/spark tables but with sub-category lists of possibilities for the tag (most: enemies, friends, complications, things, places, NPCs: ambitions, powers, dreads). There is an amazing amount of idea-sparking detail there.
There is too much detail in the system for me to want to run it with my current level of experience. Maybe someday. But there is an awful lot of useful stuff for other games, even if you're not going to run WWN.
Answers to specific question about WWN and the Atlas of the Latter Earth
Nothing wrong with WWM as a system. And I suspect both are the same system. It's just that there is so much there with fiddly (but admitedly totally optional) rules. And the setting is very good. But it's very high level. You're not running a xWN game without putting in a lot of work into your specific setting to fill in all the blanks. Bad analogy, but it's the difference between being given a wood shop and lumber or a box of LEGO. You'll make something cool out of both. As somebody quite new, the first is very intimidating and too time consuming for me.
I got WWN and the Atlas a bit too early in my RPG journey. It was way too many mechanics, at the time. How many master craftsmen vs labourers needed to built a cathedral and how long it's going to take depending on amounts of each. It just seemed insanely detailed. Looking at it now would go much better, but it still fits into the "I'll play if you run" category. The Atlas is very cool, but like the rest of WWN, it's not play-ready. It's an encyclopedia of a fictonal world you can use to create your adventures. It doesn't have ideas for a dungeon - it has info on climate, geography, culture, population densities, history, etc.