I think we have the best defaults currently possible for the use-cases Loom targets, say, more than a few thousand concurrent tasks. The cases where you might observe some downside compared to the OS (before we choose to expose forced preemption) are not in that class. The only thing to consider is whether you have many concurrent tasks or a few, and if the answer is many, the choice is simple. Otherwise, you can experiment with different implementations, but the few-tasks case is not our initial focus.
Having said that, I'm interested in hearing about real-world cases (involving many tasks, not 32) where forced preemption, and possibly time sharing or maybe another strategy, can be useful. The "accidentally misbehaving subset" is a good example, but time-sharing probably isn't what we need to address it.
