So winrates have really been smoothing out lately and outside of some issues, and I think its time some of the less useful doctrines got looked at. One that really comes to mind is USF Mechanized Support.
Overall, the WC51 is okay, but a little too pricy, but can't come too early since OKW lacks early AT and can't be too cheap so it can't be spammed against them. The M3 is terrible with only 200 HP (10 more then a kubel) and a very high price (35 fuel for effectively a Soviet Scout Car that can reinforce), especially when compared to Tactical Support's M5 halftrack. The AssEngies that come in the halftrack are also pretty meh outside of Demos and mines. Finally, the Recon ability, while generally useful, seems redundant since you're supposed to be trading up your vehicles for a major anyways.
*WC-51: Start the unit on cool-down and increase the cooldown to prevent spam/retain timing, change cost from 240/20 to 240/15.
*AssEngies: Grant them a fifth man with vet or some form of defense. Right now they're good if you need mines or demo, but are pretty lackluster as a combat unit.
*Halftrack Group: Defensive stats mirrored to M5's (HP from 200 to 320, armor from 5.4/4.2 to 28.5/20 and target size from 18 to 20), cost from 540/35 to a more manageable 490/30. Alternatively, also remove the AssEngies from the call-in and reduce the cost to 260/30 to make multiple M3s more feasible.. Either way, correctly set CPs from 2 to 3.
*Recon Run: Nice, but not needed since you'll be rushing Major anyways. MAYBE replace with Heavy Cav's Combined Arms ability so that rifles and Ass engies can really take advantage of having a supporting M3, even later in the game.
EDIT: References, so we're all on the same page with stats close at hand.
EDIT 2: Test mod
WC-51 dispatch cooldown set to 60 seconds.
Combined arms is actually on the M3 because I have no idea how to get the commander ability to change. Its got a global cooldown though so it shouldn't cause any issues.
AssEngies get 23% RA at vet 1.