So I decided last week and have found it to be a case "works on Paper"
I noticed an exchange with a BCH/USDT 50% higher than coinbase's price for BCH so eyes watering bought some as a test. It was just 50 to see if it worked. It did not. The problem was the other exchanges BCH wallet was down so i couldn't deposit it in.
Whenever you notice huge price gaps like this, there's usually an underlying reason. Usually it means a coin can't be withdrawn from the exchange. It could be due to shady "wallet maintenance" that lasts for weeks, or it could be due to insolvency where the exchange just disables withdrawals.
Since traders couldn't withdraw BCH, they were desperate to sell it for another asset (like USDT or BTC). In other words,
arbitrage is impossible. When arbitrage is possible, the price gap among exchanges should be very small because arbitragers can actually close the gap.