I've been monitoring the Steem network for a while to get an idea of the downtime. Except for a few short periods of time, it has been pretty reliable. There were some issues over a year ago but has been pretty smooth sailing since.
Once Hard Fork 23 landed and Hive was created, the reliability of Steem has been in question. The nodes and the site are frequently down and unresponsive.
I believe part of this is when Steem goes in and modifies the code to censor anyone mentioning Hive. As you can see from the recent changes on GitHub Justin Sun is actively blocking posts and even entire user accounts.
All my years of work on the Steem blockchain completely erased because I made a post about Hive.
This was because of a ninja Terms of Service change made by Justin Sun to outlaw talking about any other blockchain or any other person, although it is only enforced when talking about Hive.
There is some other fuckery going on as well I am not completely sure of. Judging by all the previous actions, I can't imagine it is good for the future of Steem.