The biggest concern I have with supply is that as I understand it, it only really considers if there are convoys reaching the army in question, it doesn't really care about actual military production. Atm, even if army goods are completely unsupplied, this has little to no effect beyond cost, meaning a sufficiently rich nation can seemingly conjure military equipment from thin air. While obviously adding stockpiles or making other fundamental changes to the purchasing system would be beyond the scope of this update, armies should, at the very least, take a significant org penalty when experiencing input good shortages.
Longer run, it's a significant design problem that fundamentally military industry *doesn't matter.* Because money is a sufficient substitute for military goods, there's way less reason than there ought to be to invest in military production. You can just use the accumulated revenue from that profitable industry to pay for the military goods at a slightly higher rate, rather than trying to sustain an industry that is fundamentally unprofitable. Hopefully the world market will help a bit since you can have an arms export industry without micromanaging exports to warring nations, but that's a band-aid. There should be a significantly military advantage for countries that can produce more arms and ammunition, *especially* as we get further into the game, and at the moment, there just isn't.
The big story of war across the period this game covers is the rising importance of military industry, the declining importance of men as opposed to guns, and ultimately the total redirection of economies to military production to win wars. At the moment, Vicky just doesn't really simulate that, and you can plausibly wage war throughout the entire game while barely paying attention to your military industry. Unlike a lot of people, I'm actually totally okay with the simplified front system being used to keep focus on the core economic gameplay. But for that to really work, that core economic gameplay needs to have *WAY* more impact. By the late game, it should be borderline impossible for a less industrialized nation to compete with an equivalent more industrialized one on the battlefield, *even given tech parity.* The Russian army was a mess in WW1 not because they didn't know how to make good rifles or artillery, but because they lacked the capacity to produce enough of them. The game in its current state cannot simulate that.