The daily counter is the usual friction. Ten rewrites is plenty on a quiet Tuesday and useless the night before a deadline, and unused days do not bank, so the allowance suits steady work far better than bursts.
Price is the other reason. At $20 a month, or $16.67 billed yearly, StealthWriter has the highest entry price of any dedicated humanizer here — though it also has the most free capacity, which is why people hesitate before leaving.
A third, for teams rather than individuals: there is no public API, no developer documentation and no blog. If you were hoping to script it into an editorial workflow, there is nothing to call.
None of that makes StealthWriter 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 StealthWriter comparison sets its numbers against ours in detail, including where StealthWriter wins.