Just because the group is based on the comic books doesn't make the mash-up a viable one. I suppose those who are fans of the Marvel comic books can relate to the group, but many of those who aren't, find it a stretch when Iron Man, Hulk and Thor are creating massive damage, while Hawkeye, Black Widow and Captain America's relatively anemic efforts pale in the melee.
The reason the park opening has been pushed back (and may be again in the future) is because they are having problems that will take additional time and money to deal with.
As for the park being bad/on time or good/late, that will not be determined until 2017-18. The first year after opening (whenever that is) will still provide a good attendance number as everyone will be looking to experience the park just to find out what it's like. However, it's the following years that will show whether it is actually successful.
So, maybe by 2020 we'll finally get an inkling as to whether the cost and effort were worth it.