USAA do not control over interest rates nor how dealers price their vehicles. I saw a post from someone who stated that the car buying experience hasn't changed and it won't no matter what service you're using. I tend to agree.
Those who have posted poor reviews are expecting too much. Special rates and pricing are administered by vehicle manufacturers, not insurance companies. So if a dealer is not running special pricing or interest rates or doesn't need to move a certain vehicle, USAA are not going to force them to bend the rules. This especially applies to limited production models. You're not going to get some secret "under the table" discount through this or any other organization on vehicles that aren't typically offered at bargain rates.
Those who have posted favorable reviews just happened on a make/model/year that the dealer was willing to bend on. The better deal has nothing to do with USAA's influence.