Bought a 2016 refurb MBP, in 2016. About $500 off (CAD).
Ended up returning it - it had occasional instant reboots, like when I would plug in an external monitor. Not many, say 2-3 over a 2 week period, but I was concerned I was getting some kinda unusual lemon - after all someone did return it. Also some glitchy keyboard but that might have dust and the notorious butterfly keyboards.
A Genius hardware test didn’t show anything.
So, I called up Apple to return it (as if it was new, 2 week period). No questions asked whatsoever, no pressure, and they gave me 2 weeks to ship it back. So I had it for 4 weeks. Full refund, not an exchange.
It can also be worth it, depending on model and features, to buy used. Right now, I have a 13 day wait for a new 32GB ram MBP 2019 in Vancouver. Or I can get a refurb (again around $500 CAD off) refurb in 5 days. Same thing in 2016 — I wanted a 512 SSD which had 10 day delivery wait when new (Apple Canada used to only stock 256 SSDs).
Bottom line: it saves you some (not a lot on current models) money, but it’s not totally foolproof - I got a glitchy machine. On the other hand, Apple’s return policies are sufficiently lenient that it’s not risky at all.