I have been playing Challenge Mode (all grades are 20/80) for a few weeks now and found some exploits in the AI.
I love this game and understand the complexity of it makes it impossible to match real life so don't take this in too negative a light,
1) The Jo Adell exploit
This must be common knowledge. Why is the AI not valuing prospects based on max(potential rating as pitcher, potential rating as hitter)?
2) The Andrew Miller exploit
This has nothing to do with Andrew Miller himself. Sometimes I see international free agency posted a 80-stuff, 60-movement, 70-control reliever from Japan/Taiwan, who has only two pitches (both 70-80) but 50-stamina (basically an Andrew Miller). As relievers in absolute overall rating they are about 45/45. I sign them to about 5 mil per year and then change position to SP, they turn into 80/80 SP. When I keep them, they sometimes perform like true 80 SP but they are volatile in-game and perform like a middle-rotation starter over the season (due to having only 2 pitches). The best way to use them is trade them as SP for a 5-star prospect (60+ potential), and the receiving AI immediately turns them back to RP/CL and use as such.
3) The Pablo Sandoval exploit
Cannot trade for that 5-star prospect (60+ potential) on the Red Sox you always wanted? You can lower their demand/price by bundling that 5-star prospect (60+ potential) with bad contact. I prefer a bad contract that is expiring this year or with an team-option for next year (thus also expiring this year). On the Red Sox this is very easy to get. Allen Craig in 2017, Pablo Sandoval in 2019, Rusney Castillo in 2020. Yes may all Red Sox teams be plundered in OOTP18 saves across the globe
In short, the AI is not valuing properly the negative value of a bad contract that is expiring (or have a team-option that 100% will be declined). An expiring bad contract appears to be as bad as an Albert Pujols type contract.