My dad was such an engineer doing sales, of industrial components. Grew up on a farm, engineering degree, very honest type churchgoer and family man, and in his spare time DIY projects like a classic engineer type. I'm sure he'd know when something would or wouldn't work, and would candidly tell the customer about any problems or risks. (In this case, maybe honest as much as an engineer personally bothered by design flaws.)
I've also seen a different kind of engineer in sales, where they're paired long-term with salespeople. They sit in on sales meetings as a technical expert, and also do things like customizations and integrations. I suppose the presence of the salesperson helps suppress the engineer's inclination to start riffing on every flaw, but the pairing retains the engineer ability to help the customer be successful with the product.