Assuming that the system is in working order with all nessecary hookups/controllers, I really think I'd be hard pressed to say 'no' to a system. Off the top of my head, I might turn down:
A 5200- I actually had one a couple years ago & sold it. It didn't have the controllers/hookups and I didn't realize it was so expensive to get said bits. I might take one now, but given the size of the unit & the fragile natures of the controllers meaning I'll likely be in the same boat as before in short order... I might pass.
A CDi- once again, it's a big system that would be hard to store, and I've not seen one game for it that looks remotely fun to play. On top of that, the games are somewhat rare & therefore tend to be on the pricey side to boot. I don't want to spend $30-$50 (or more) on a game in the hopes that it might be somewhat enjoyable.
A Hyperscan- I typically avoid licensed games like the plague, and this thing is ALL licensed games.
A Famicom- ok, unlike the others, this one certainly has fun titles I'd like to have and it's not prohibitively big... but considering I could play most of those interesting games on my top loader with an adapter, I'm not sure I'd be willing to deal with the hardwired cables & fiddliness of hooking up old Japanese systems just to use a famicom specifically.