Request metering is the usual reason. People who edit in small passes find a 75-request month disappears into a fortnight, and the fix is a habit change rather than a plan change.
The 400-word cap at entry is the second. Long documents have to be broken up, and rejoining separately rewritten blocks is where consistency goes.
The third is discomfort with the claim. Some people are fine buying a tool that promises undetectability; others realise afterwards that they relied on a promise nobody could keep, which is a worse place to be than never having been offered one.
None of that makes Twixify 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 Twixify comparison sets its numbers against ours in detail, including where Twixify wins.