Will bad play in one cruise line's casino put booked trips at risk?

Potentially. This is pretty rare and is determined on a case by case basis. Most players we book are consistent when it comes to their play. But anytime you don't play on a trip it will affect future offers. If you don't play in Cruise Line A's casino and you have 2 more comped trips booked through Cruise Line A after that trip, then they may look at your future trips and reevaluate the comp. They'll look at how big the investment is on their part.  There's not like a hard and fast policy on it, but it could be concerning and would definitely affect future URComped offers if play dropped off.