AI don't use some units at all

I had that problem before with Paratroopers, but in some patch AI started use them. Now they don't use American tank destroyers though - I never seen them use Jacksons or M10s in battle, even though they used them before without problems. AI loves to build Shermans and halftracks (I added M3 HT for americans) but they never, ever build TDs. Is there any way to fix it? I don't know, maybe there is some priority list or something for AI?

Indeed, there is a way. You'll have create a custom preset in the ai_economy section and then add it as override in the ai_economy_group section (remember to set active_by_default to true)

I have to warn you though: This area is largely undocumented and getting the results you want to will require a lot of time and effort in trying out different settings, always keeping the factor of chance in mind. You'll also probably want to add an entry for every unit in the USF faction to be able to better balance them against one another.

Under "ai_economy/overrides/[yourpresetname]/squads", add a new "squad_utility" in case the unit you want to adjust isn't already listed. For "blueprint", set the sbps file of the unit.

As for "per_unit_scale", I'm not 100% sure, but countless hours of testing lead me to the conclusion that this controls how many of one unit the AI will produce at all. If you set it to "0.05", countless TDs will swarm the field if the AI has the resources. Set it to "0.5" and over the course of the match you'll most likely see two at max before the AI chooses to build other tanks instead. Works different for Infantry. But again, just my guess at making sense of the variables.

Finally, the "utility". Pretty straight forward, it tells the AI what the unit is worth. Extremely inconsistent, probably a modifier to some hidden (?) base value of a unit. You'll need to experiment. Random offset should be clear.

In any event, there are some units that the AI will spam even if their utility is near 0. Such as the M4A3 Sherman and Halftracks that can reinforce. You'll have to find the right balance of utilities for your mod.

I hope you can make some sense of this.

Oh man, sounds like hours of tedious work. Also there aren't any presets for USF and Brits, so I'm not sure if it will even work...

It will work. I did this numerous times. You'll need to create the sets of overrides (look at the ToW presets for examples). But yes, it will probably cost you many hours of testing and tweaking until the AI performs in a way you'd like.