I gave this mod a try and I think there are a lot of cool things in it. I like the personnel and materiel resources. There are a bunch of good maps and scenarios. Ever since CIV 2, I've always enjoyed WWI / WWII scenarios in CIV games.
So this is what I've played so far:
RED v.4 Data
RED v.31 WWII Edition
and the Xtreme small scale mod for v.31
I've been playing as UK in the Europe 1936 scenario, so here are a few comments I can make about it (and probably a bit about the scenario balance overall):
-I strongly agree that air units need to be tweaked some more, ie interception should be a bit more effective.
-It would also be nice for strategic bombers to actually be able to damage infrastructure.
-If bombers can do infrastructure damage, it would also be necessary to repair that damage. I think we should be able to have workers, though their cost to build should be very high (maybe around 800, which is more than the cost of a battleship), and the time taken to build or fix improvements should be long. I'd suggest something like 25 or more turns to build a railroad.
-Air recon as was mentioned I also think is essential.
-Air units should probably be able to do more damage to naval units, but see my point below.
-Destroyers with their intercept should be able to intercept indefinitely. Ie, they shouldn't just do damage to the first unit that comes by them. The amount of damage they do back is pretty limited anyways. I think their AA guns should keep firing for each air unit that attacks them. You can't stack naval units, but you can essentially stack air power on the same square, so I think the navies should be able to defend themselves against this properly.
-My biggest gripe for playability so far: The inflation of military unit upkeep costs seems out of whack. I've gotten to August 1 1940, so far in this scenario, and as the UK, my military upkeep is through the roof already, giving me a massive negative gold balance sheet. Unit upkeep per turn per unit started out below 1 gold (I'm not sure what exactly) but now its almost 3 gold per unit per turn. Since there is no ability to grow your economy, ie cities don't grow, the only thing you can do is build banks, once thats done, you have no ability to generate more cash yet unit costs for the same units keep increasing. At best, the unit inflation should be eliminated altogether, at worst, it should be severely reduced. Basically I feel that at this point in the war, as UK, I should be spinning out as many ships and aircraft as possible, but instead, I'm actually having to disband units, even with all cities on wealth production, just to keep a positive balance sheet due to unit upkeep inflation. This is basically a game breaker as far as I'm concerned, and should be easily fixable.
-I get sporadic crashes, always during the AI turn. I always save before the end of my turn, and reloading and trying to end the turn again has always allowed me to progress my game when I do get a crash.
-As UK, I was unable to see into lands of players like France, US, even when I had units that should have been able to see into those lands. When players like France did actually become allies, I was required to save and reload the game before I could actually see their lands. They remained in fog of war as long as I kept playing the game without reloading.
-When load this mod, sometimes I get an error that it can't find a particular background image of some sort .dds file. When this happens it just means that I don't get one of those nice background images when I load the mod, but the game still allows me to click to start a new game or load a saved game just fine.
-I think Artillery is underpowered for its cost, range, and how it compares to air power. I think doubling its ranged strength might make it a more attractive option given its cost. I think big guns like this should play an important role in a WWII scenario, but they almost feel irrelevant if you compare them to aircraft which have much larger range, and can stack their attacks on the same unit much better. I think your unit should feel it when they get pounded by artillery, but artillery should have a mobility problem, which is where air power really shines.
-With the option of limiting embarkation to ports, there is a problem: You can't use allied ports. I really like this option, but as the UK trying to defend northern France from the German invasion, I'm unable to actually withdraw my troops out, once they have landed in France. The historical withdraw from Dunkirk is impossible with this option on. I'm sure there are numerous ways this could be tweaked or fixed, while still having the option on. Ie, you can embark if you're beside an allied / friendly city with a harbour, or you can embark if you have a naval unit in the coastal tile beside the unit, etc etc.
Having said all of that, I think this mod has a lot of the big picture stuff done quite well, and its a very fun game to play. I also love all of the different specific units and graphics, as well as the much improved unit graphic sizes.