...theres such disparity between characters in this game, that the content you make to challenge some will be impossible to others, and could be trivial for a few...
Well, in my module building I go to a great deal of effort and trouble precisely to prevent this sort of thing from happening, and provide encounters that are reasonably challenging without being impossible for any reasonable build, so I am naturally a bit leery of anything that is likely to make this task harder. Furthermore, if character building options become so extensive that any sort of balance becomes impossible, they can actually end up providing less variety that is actually meaningful, as either most players end up using only a handful of the most OP classes and ignore everything else, or the designers find it necessary to make all encounters easy and generic lest some weak class prove unable to handle them. Content (combat related or otherwise) tailored to specific classes or other character features becomes harder to include if there are too many of them, again with the result that the game becomes more generic and boring, as every character tends more and more to be treated alike and not assigned tasks that call for any unique capabilities to actually be used.
In any case, I am not saying that a wide variety of character building options is a bad thing in general. I like having a lot of character building choices too, provided it is not carried to a ridiculous extreme. What I am objecting to is
retroactively adding a lot of new classes, races, spells etc. into a game most of the content for which was not designed with those particular build options in mind. That could potentially introduce a lot of problems (and not just to combat balance but to other things like class-specific quests, providing proper gear and interesting items, etc.).