I used to use 1/8" axles all the time too. When I first started manufacturing these cars, I thought about switching my personal McPappy cars over to 1/8" axles. But then I realized that the 3/16" are much lighter as they are aluminum. 1/8" axles are usually steel. I've only found 2 titanium 1/8" axles. Lunsford's titanium axles were a bit too short and the outer bearing rides on the threads (not centered well). I ordered some T2C titanium front axles and they gave me steel, probably on accident, so I can't speak for those axles.
The only downfall that I've heard concerning 3/16" bearings is that they are less durable. Probably because they use smaller balls and can dent the inner race easier. Personally, I have not seen this to be the case. The last few years I've been racing with quality drivers and I don't hit things that often, so maybe I'm not the best judge of their durability. Lately, the only bearings that I find to go bad often are the rear bearings.
I believe it's more important to save weight everywhere possible on the car so that we can run as much tungsten as possible. So I've been very pleased with the light 3/16" axles. You will not need to use any shims as the McPappy locators will be the better way to adjust track width. As for the nut, I hear ya. That plastic nut is a pain in the butt. I push it on while using the nut driver and it goes on pretty straight most of the time. I also take it off and use an Xacto blade to trim off any plastic that bubbles up after you thread them on the first time. Most people do not use shims with these plastic nuts.
These plastic nuts are also an alternative.
Personally, I use metal 5-40 locknuts like these:
I was lucky to get some aluminum ones from a friend who had them for 20+ years. It's impossible to find aluminum 5-40 locknuts these days. If anyone knows of any, let us know!
Merry Christmas everyone!