Over the years, for myself and my wife, I've purchased three of the old white plastic MacBooks, two 2012 Mac minis, a 2019 MacBook Air, and a 2019 iMac -- all refurbished. I also purchased a new 2000 G4 PowerMac, 2004 iBook, 2008 MacBook Pro, and 2015 MacBook Pro. The refurbs looked every bit as new as the new ones. The PowerMac died after 10 years of use, including many upgrades, so it had a good, long life. The 2008 MBP experienced the well-known NVIDIA card failure, and Apple repaired it. Every other Mac on the list is still working fine, or was until I sold it, gave it away, or put it in the closet because it had outlived its usefulness.
The reliability of Apple products is why I don't buy AppleCare, the exception being for the 2004 MBP. That practice has saved me thousands of dollars, so I'll come out ahead even if I do eventually need a repair on a product that would have been covered under AppleCare. But that's a topic for another thread. Anyway, I don't see a problem with getting a refurbished Apple product under warranty or AppleCare. If they were scratched, scuffed, dirty, or had other visible signs of wear and tear, then I'd be unhappy -- but, as others have pointed out, in most cases, the product you bring in for repair is no longer pristine, so a refurbished replacement is going to look newer than what you brought in. Since I've never had a refurbished Mac fail during its useful lifespan, I don't know in what objective way refurbs could be considered inferior to new products.