I have found what I believe to be a discrepancy between the “old” and “new” form extensions.
The “old” forms don’t let you set conditional logic based on the values of a “rollup” field type. The “new” forms let you choose a “rollup” field type for the conditional checks, but the condition does not have any effect. This makes debugging the conditional logic very confusing.
I think the best implementation would be to match the behavior of the “old” form, where it doesn’t let you select a “rollup” field type for conditional logic.
I have a screen recording here that shows the behavior.