Explanation
What makes a testimonial persuasive is specificity rather than enthusiasm. Unqualified praise reads as fake even when it is genuine, and the versions that work almost always open on a reservation: somebody saying they nearly did not buy it, and then why they did.
Using a generated presenter to deliver a real customer's words is a recognised practice called a dramatisation, and it requires disclosure. Generating the review itself is a fabricated endorsement, which is a straightforward advertising-standards problem in most markets rather than a grey area.
The common mistake
Polishing the customer's language. Reviews convert because they sound like a person, and editing one into marketing copy removes the exact quality that made it worth using.