The biggest issue with infantry oil is that it obsoletes the entire line.
Who in their right mind would ever use precious oil on an Infantry unit:
70str 2 move
When for barely 11% more production, you could have
TANKS.
80str, 4 move.
Because mounted units have no city combat penalties, and still get terrain defense bonuses (unlike past iterations of civ) you are literally throwing oil away on infantry. Tanks are straight up twice as fast and hit 50% harder.
Put it another way: a tank unit is an infantry corps with +2 movement.
Now, the mechanized infantry vs modern armor isn't as lopsided. The Armor only has a 5str, 1 move advantage and it does cost 680 prod instead of like 650. (A 5% increase.) But then again, it is straight up a better unit.
Also: tanks & ModArmor hit an antitank/modern AT unit at parity since they are 10 points stronger in both cases. That's right kids, the Antitank units don't actually beat tanks in combat, only on cost. A modern armor unit is a whopping 17% more expensive than an antitank unit.
To put this in perspective with early game warfare: Swordsmen and legions both cost identical iron. Legions have +4 str and cost 22% more for this privilege. Legions are actually overpriced based on tank/infantry.
Okay, a concrete example since I know people will mention tank and infantry are different unit classes.
A sword costs 90 and takes 20 strategic resources. A hetairoi costs 100 and takes 20 strategic resources. This is exactly 11% more, just like the tank is.
Therefore, it would be internally consistent if Macedonian Hetairoi (a heavy cav unit) was buffed from 36 to 46 strength, and 4move, and this would be considered balanced against a swordsmen (36str, 2 move.)
This is obviously not balanced.
The solution is to actually just increase the strength of infantry, antitank, and modern AT units by 5 so they are 75 & 85. But they've never addressed modern combat & I have to assume they are aware after 2.5 years. If you plot out the units' strength it's very noticeable that these 3 units deviate from the trend line and the +10str/era rule.