Fuel to Ammo conversion is very unfair?
therefore, the MP to Fuel in Soviet industry is immensely unfair?!!
Soviet industry is extremely unfair to the soviet player actually as you have no control over conversion, benefit is not instant and disadvantages heavily outweighs advantages(have fun arguing that, the popularity and presence of industry doctrine in the games speaks for itself).
With CAS conversion its 100% your call, you don't need it, you don't use it, you need it, there, instant reward!
So you have picked a really bad example to compare.
If one resource conversion ability is plain OP, its this one as all others give up something that can't be made up for or are counterable or dependent on additional factors.
Industry will punish you sooner or later because of constant penalty that fuel does not make up for.
Opels can be destroyed.
Luftwaffe supply drops can be stolen, require your unit to be there and are extended over 1 minute.
OKW conversion is dependent on territory capped, doesn't grant instant reward and you are slowing down your other resource income akin to industry, but you can switch and turn it off at will.
CAS conversion(which is also present in other doctrine, but isn't as imbalanced there) is simple click for reward with little choice as you are still able to tech and get armor reliably.