The 500-word request cap is the usual reason, and it is specific to the entry plan. People buy the cheapest tier, discover their documents do not fit through it in one pass, and find the fix is a different plan rather than a different habit.
The second is the absence of anything free. There is no way to re-test the output after a model change without an active subscription, which makes it hard to tell whether quality moved or you did.
The third is that the pricing page states very little beyond price: no rewriting language support, no API terms, no accuracy statement for the bundled detector.
None of that makes AI Humanize a bad tool. It makes it a tool with a shape, and the question is whether that shape matches your work. Our full HumanFlow vs AI Humanize comparison sets its numbers against ours in detail, including where AI Humanize wins.