Briefly, code 100 is heavier than any mainline rail ever laid, anywhere. That famous PRR 152 and 155 pound rail would be code 93 if anybody ever made any. Code 83 is a good approximation of heavy mainline rail - used by Northeastern lines and coal haulers since the 1920s, and by almost all the Class I roads today (There's an industrial area near my home that was laid with 132 pound rail dated 2000...)
Except for those unmodified NEM flanges on some older European-made cars, code 83 will handle anything you run on it. Even most of the NEM flanges will work.
In (somewhat) defense of Code 100, it is cheaper than code 83, and works just fine where the sun will never shine. That's where my original stock of Code 100 flex went, laid in the netherworld along with some stick rail, sectional track and other odd ends.
I experimentally laid one length of Atlas Code 83 in the Netherworld and discovered that Atlas universal rail joiners will connect it to Atlas code 100 without any special modifications or conniptions. These aren't transition joiners, just the regular kind. I did weigh down the freshly-caulked rail with a flat-bottomed weight to assure level rail ends - but I do that for every length of rail I lay.
Availability of specialwork is one thing that has zero influence on my rail height decisions - I can hand-lay mine with any height of rail, down to and including code 25 (mine car tracks.)
Chuck (Modeling Central Japan in September, 1964)