Yobit AFAIK is "not" a scam, though there have been many many cases of yobit scamming its users with large amounts of BTC or money.
You can read about every exchange a lot from others on internet. Problematic exchanges are down for now, good for us.
Ummm, I wouldn't say so. Essentially all exchanges have had reports of scamming its users, so approach every exchange with caution, and also make sure that it is an exchange in your country. This helps ensure that if anything goes wrong, you can go to their front door and complain. Its much easier to scam overseas users than those that are literally at your doorstep.
Stay away from yobit for now, and choose a more appropriate exchange like poloniex and bitfinex, etc...
