I've traded on mercatox for quite a while, and didn't have a single return yet. So i feel pretty safe, working with it.
It's good to know that you feel safe with them because from what I've been reading with the reviews on mercatox, a lot of traders aren't confident and is annoyed with them. And this isn't only for them but for other exchange as well.
It is not recommended that you have so many accounts, it seems that you are a new trader, so it is best for you to concentrate in a single platform so you can get used to it and you avoid making mistakes when you trade, once you get a lot more experience you can use several accounts in different exchanges and find the one that you like the most.
He probably is a type of trader that wants to try a lot of things on different exchange. And having a lot of accounts means he has experienced the differences of those exchange that he has an account. Can't assure with any exchange that your funds are safe but use only those exchange that's credible and has a good track record like binance.