I'm not @transisto (obviously lol) but just butting in to say I've been using Bittrex for some time myself. In my opinion they are safe, although their withdrawal fees may fluctuate from time to time. It's also a place where you might see P&D's happening, due to the sizable number of sh*tcoins they handle on the platform.

Exchange wallets going offline for maintenance is not uncommon, especially if there's a fork, or wallet software is being updated or malfunctioning, or a daemon is broken. but it's not something that happens frequently, or for every coin in my experience. Ethereum wallets at one point some months ago were going down every week it seemed. I remember transferring some Ether there just before wallets went into maintenance. wasn't impressed.

But as for Steem wallet going down for maintenance, Bittrex probably had a good reason. last thing you want is a wallet daemon spazzing out and eating peoples Steem or sending it off into the void instead of your steemit wallet.