As long as the A/F ratio stays within the required limits along with timing then the engine can't experience problems from progressive timers as it's seeing the same A/F all the way thru the rpm range.
Actually applying limited amounts of Nitrous at lower rpms with a progressive timer is less detrimetal then if the rpms are too low and vast quantites are injected.
Remember that N/A engines pull the amount of Air and fuel it requires as the rpms increase. But with Nitrous, you put what ya put the entire rpm range.
