I've heard people criticise Capt USA 3 for:
#1: Not having a resolution -- there actually is one
#2: None of the main characters dying -- there's reasons for it
#3: Thor and Hulk's unexplained absence -- again, reasons

1. I think a lot of people wanted to see either Team Cap or Team Fe win, but both sides lost. The real winner was Baron Zemo, because his primary objective was to divide the Avengers. Make them fight themselves and self destruct as a unit. He has achieved this objective. Although the two sides are no longer warring, they are now a house divided.

2. Continuity. We know that the MCU is building more movies for the next couple of years, and as a result their main players are essentially locked and placed on a "do not kill" list in order to maintain continuity for those future movies. Captain America 3 is only the first of the MCU's Phase III movies, with other Phase 3ers including Dr. Strange, Guardians of the Galaxy II, Spider-Man: Homecoming (and 'homecoming' was one of Bucky's activation words), Thor: Ragnarok, Black Panther, Ant-Man & the Wasp, Capt. Marvel, Avengers: Infinity Wars 1 & Avengers: Infinity Wars 2. Marvel most likely wanted all of the Avengers to at least survive to appear in the Infinity Wars films.

3. As above, Thor and Hulk are locked into events for the Thor: Ragnarok film, so they were excluded from Capt. America Civil War in order to prevent continuity conflicts with the Thor 3.

One thing about building a big intertwined cinematic universe is continuity maintenance, which does restrict film makers in terms of what they can and cannot do with characters. This is why Christopher Nolan refused to tie in his Batman movies with any other DCU characters like Superman, and also because he wanted his story to focus on a single protagonist (or two with Robin's inclusion in The Dark Knight Rises). This is something that I wish Michael Bay would pay closer attention to.