The site became mainstream while still being extremely vulnerable to manipulation by a very small pool of individuals. They've done next to nothing to address how effective and rampant brigading is (among other tactics).
What's frightening is that groups like politicians, unions, and extreme right-wing groups could very easily use this technique to quickly and pretty effectively promote or quell discussion.
I just recently saw an example of where this might have happened. Very suddenly a comment in a discussion I was following - what initially was a popular up-voted opinion (i.e discussing police brutality) - got down-voted into oblivion (from +5 to -25) in less than 5 minutes. The comment was deleted soon after - probably to protect their score.
This kind of quick/easy manipulation by bots can't be easy to detect/deal with yet I fear it may become mainstream.