Where Suicide Squad, or where the DC cinematic-universe fails in general is that they're trying to tackle these larger than life DC comic stories without establishing a universe.
I hate making this comparison, but if you look at Marvel, they've been building their cinematic-universe since 2008, so that's 8 years now. They started with Iron Man, followed by several solo hero movies, slowly building the tone, and sculpting a feel for what the Marvelverse is. Then after enough build up they were able to do the Avengers which was wildly successful. Now DC is trying to recreate the success of Marvel's avengers without the years universe building. Batman v Superman tried to compete with Civil war, which was in works for years, and now we're going to get a Justice League movie followed by solo hero movies. If you ask me, that's ass backwards thinking.
In terms of Suicide Squad, there were several seperate issues with the movie (editing, horrible villain, clusterfuck of origin stories, shit villain, etc.), but if DC had previous experience with smaller solo hero/villain films, then they wouldn't have ran into so many problems with such a big title.