Purigen has actually become quite a debated media especially when used in a planted aquarium. As it is how purigen works is never greatly described by Seachem which makes one wonder why. It's simply described as being able to absorb organic waste such as [(NH2)2CO], uric acid [C5H4N4O3], and creatinine [C4H7N3O] before it's able to be turned into waste such as ammonia (NH3), which is then converted to nitrite (NO2) and then subsequently to nitrate (NO3).
It's also stated Purigen should not be used in a tank before a tank is cycled. This is because by absorbing organic waste Purigen removes what's needed to establish a good beneficial bacteria colony. When you consider that, one might wonder then what happens when a beneficial bacteria colony is established and Purigen is added. If Purigen is added won't it remove the very organic waste that's maintaining the established colony? And if it does then won't the bacteria die since what has been sustaining it is being removed?
Then add plants into the equation which are also in need of these organic wastes to do their job. Without these organic wastes wouldn't the cycle be broken for the plants as well?
So now that your system is free of organic wastes isn't that desirable? Since if Purigen is doing it's job so well it's starving the BB? Well maybe for awhile but then you have to consider since Purigen doesn't involve a natural cycle as it becomes exhausted won't it's efficiency go down? And as it goes down will the BB have enough time to recover before total exhaustion of the Purigen causes a Spike?
That's what more or less is what's being debated from what I've seen with about 50% for Purigen and 50% against.
To me I'd rather go with as natural a cycle as I can for one reason. Nature/life adapts. As your bioload grows say from adding fish, fish growth, over feeding or some other event a natural cycle will grow to compensate, it will grow and shrink as needed. Alage in a Scrubber will grow faster as will BB production in response to the changes. Albeit not fast enough if you do things badly but you can easily plan around it. While Purigen can't. It's starts off at one efficiency and that efficiency continues to degrade as it gets exhausted which I imagine would be accelerated by increased bioload.
