A little insight from an old fart:
I always preferred using 1020 DOM or 1026 CDS (which is actually pricier than moly) for the main cage due to their ductility.
Both are much "tougher" due to their malleability.
Its all about the point at which materials begin deforming, to the point at which they tear or ultimately fail.
Yes, moly has an ultimate tensile strength 10-20% higher than the low carbon steels, but its elongation properties are much lower than 1020 or 1026.
Yes, .15-.20% carbon content DOES make a difference in regards to toughness.
Look at it this way, landing on the roof at 100 mph is better with cro-moly IF it's a one or two hit crash.
Barrel rolling at 100 mph, with multiple roof impacts becomes troublesome with 4130 because it does not deform well without tearing. That deformation is analogous to a crumple zone in the front of your car, or the foot box tearing away in an open wheel racer.
Add in the effects improper, or lack of post weld treatment of medium carbon steels (4130) which STILL is rampant in today's age, and you have a recipe for disaster.
For the record, 4130 roll cages are NOT heat treated like a 4140 spindle may be, they are stress-relieved by either high frequency vibration methods or carefully applied post weld heating and cooling processes.
I remember talking to Bill Stroppe (yep, I'm THAT old!) at the Parker 400 right around the Rough Riders campaign.
There was alot of monkey see sh** going on then.
While I was stress relieving ALL of my 4130 weldments at 1075-1125 degrees in the dark of night with a rosebud torch, he had this cool "vibratory" platen that he welded his chassis on. Done. No heat needed. Just jostle the ATOMS (not molecules) around at the proper frequency while welding and presto!
Sorry I jumped off topic.
But seeing lay-mans talk improper materials tech is like fingernails on a chalk board.
C'mon guys. At least you have the internet.
I had to go to a library to learn.