Used mechanical equipment can be way off spec and have dirty and worn heads. You can usually tell how well it was taken care of by looking at it carefully and of course recording and playing back on all channels (if possible) before buying it. If the owner was a smoker (sniff the thing, you'll know) then particles could be in all the pots and even in the head gap.
New is always safer if you're buying from a reputable shop that has a guarantee (like, if it doesn't work they'll exchange it instead of telling you to go back to the manufacturer). If money is tight and you just gotta have 8 tracks, used may be a good tradeoff, but if you know really nothing about multitracks it's easy to get burned.
