The great thing about the Avengers-Franchise has always been this winning scheme:

1. A range of very different characters like Thor, Spidey and Strange, clashing together in witty and funny dialogues that showcases just how different they are. Spidey cracking jokes about deep philosophic insights of Strange, while Thor asks about beer.

2. Incredible fight scenes

3. Both may be linked by a variety of different narrative elements, from film-noir/detective over comedy to splatter.

It's an easy base recipe, that may be filled with any narrative strings, so long as they are intriguing.

Problem with the Avengers movies and the finale in particular is:

The winning scheme is NOT used and the narrative underlying is poor, at best.

They picked parts of a variety of comics (like the "Thor get's irresponsible and fat"-plot), which is ok and linked them very poorly together. It's a series of short plots, loosely tied together with no red line (other than "Thanos is the Endboss").

Secondly, the acting is poor, apart from Iron Man of course! Even Chris Hemsworth and Scarlett Johanssen do a really bad job at depicting their character. The digitally animated raccoon is a highlight in terms of acting.

There is only ONE AVENGER-TYPICAL FIGHTSCENE, which is great, but does not make up for the rest.

Foot-Note: As a Non-American, the Glory-Halleluja-Murica focus on Cap is too much. If you're patriotic enough, you'll love it, of course.