Reads your sig... i get it.
As long as you are plain better than your opponent, you can defeat with whatever. Mechanized is NKVD bad, thing is USF is less reliant on commanders than SU.
WC51 can help you out quite a lot as long as you use it properly and with caution. You need to refit it at the right time.
AssEngies in Halftrack are pricey but you can get them in case you are floating mp and refit the halftrack if you like. You can also keep it, but its risky.
Recon plane is very good for its cost, and the arty busts okw trucks effectively.
The whole beauty of this doctrine must be refit, because you can exploit the USF's light vehicle advantage and then refit it for the majority of the spend resources, enabling you to have a very strong early to mid game while boosting your transition into lategame at the cost of your outdated vehicles. You can also always refit a Jackson for a Sherman and vice versa in the lategame.
Mechanized can be really good, its just very special and niche, and not everybody is ready to deviate from the same meta we've had for the past year.