So, I'm going to end up double posting it looks like, because as I was getting dinner I realized some implications of the math that is just weird as hell.
So, let us take a unit of 399 veterans and a Warlord from Volo's guide. A seasoned unit led by a legendary commander, not an army mind, because 400 men does not an army make, even back in those days, but an impressively large force it still is.
Using the BR calculator the CR 3 Veterans are worth a total of 798 (399*2) and their commander is worth an additional 18 (CR 12)
For a grand total of 816 BR.
This means when they attack they roll 1d20+816... which realistically means that any group with a BR less than 810 stands almost no chance of winning, because the swing is only 20 points. Even a group with 800 BR is going to have a tough time of it, because if they lose a roll by 10 points they lose 5 points permanently. At 796 an army might as well surrender, because they will never win.
To win with a group of giants, calling back to the discussion up above, they would need to be...
Wow, they can't. Even if we take Storm Giant Quintessants from Volo's Guide, at only 44 creatures in a unit they rank in at 704 BR.
The creatures composing the unit have to be CR 19 to be a group of largely CR 3.
Meaning you need 44 creatures like the Balor or Pit Fiend to even have a fair fight against this army (when it turns out you can probably conquer the world, I'll let you be an army with 400 men)
Here's the real kicker though, let's say our enterprising warlord gets his hands on some warhorses and decides to mount his army.
They lose 481 BR.
Being mounted makes them Large creatures, meaning their can only be 100 of them in the unit. 99 Veterans (297) + 1 Warlord (18) + 100 Horses (20 [CR 1/2 means 1 BR for every 5]) gives you a total BR of 335.
So... to recap. Range is worthless because the enemies can reach you before you can shoot, and a unit of Infantry is more than twice as powerful as a unit of Cavalry.
Oh, and CR 3 humans can invade and destroy the Entirety of the 9 hells and Abyss, unless you stop using these rules and use the normal game rules...
I hate to say it, but these rules are entirely worthless I think