Something fails or succeeds if it accomplishes its objective. An evaluation of success or failure is going to be off-base if you don't know the actual objectives.
You may say that a soccer team failed if every player on a team didn't have the opportunity to kick the ball. You would be off-base if the objective that mattered to the soccer players was only that some combination of their players scored more goals than their opponents.
In evaluating success or failure, you are arguing about objectives.
But explicit objectives may not be the end of the story.
Regardless of what the objectives are, an event may be a success if the net result was positive. If you intend to create a bacteria, and you fail but end up creating an anti-biotic, you have achieved a success. It may be an accidental success, a serendipitous success, or a an anticipated, unspecified success which is the inevitable result of creating a good framework and expecting to yield some kind of fruit.
Even a failure is not a failure when it is a necessary first step. The first layer of bricks that make a wall is not a failed wall. It's a necessary part on which to place the next layer.
Regardless of how well the objectives are met, lessons can be learned and used to create a better attempt the next time. If these lessons are learned, and the next attempt doesn't suffer from the same problems, then the attempt is a success on a certain level.
Who's Keeping Score?
Lastly, if you have access to details regarding the positive and negative aspects of an event, a label of "success" or "failure" is entirely superfluous.
Who cares whether you call something a failure or a success? Is there a medal? Are there investors sitting around waiting to invest money, but only if the event is "a success"?
An endeavor can be a success if you simply feel good about your efforts. Simple enjoyment isn't a measure of success. When I was a student, I enjoyed when a prankster hijacked a chemistry class and the teacher could no longer teach. That didn't make the class a success. But trying hard and getting better does.
Did the JBlogger conference fail or succeed? It really doesn't matter, does it? Let's do it again, even better.