One big problem I had with the film is that Sam is never really the "underdog" that they tried to make him out to be. They keep mentioning him not taking the super soldier serum. He's not as strong or fast as Steve Rogers or the super powered villains, yet he never has any real problems with any of the bad guys he fights. He can take on whole groups of armed & trained men, fight hand to hand - sometimes without the shield, sometimes without even the Wakandan super wings - and beat anyone without breaking a sweat. If he gets injured, he's never really affected by it. He doesn't get tired, doesn't really need medical attention (and a "kevlar weave" shirt wouldn't really help that much).
If the character of Sam Wilson is anywhere close to the age of Anthony Mackie (who is 46), Sam is probably at least around 40 in this film. So a 40-something with no super powers other than some mechanical wings can do things that even Steve Rogers couldn't do. He looks to be in good physical shape, but by no means does he look like he's some elite athlete.
They should've shown him getting beat up more. Like in "Rocky" movies, Balboa gets the **** kicked out of him a lot before ultimately triumphing, and there's a training montage showing him working to get better. Sam Wilson needed something like this, but they wanted to make him into some idealistic image - a "normal" man that can be the symbol for American "can do" effort. If you work hard, you can acheive anything. As Bucky told him, "Steve gave people something to believe in. And you give them something to aspire to." But they didn't ever show him working hard, or working to be that aspirational figure at all. He just does things than no one else can do without explanation. So it just feels unearned.