Controversial opinion, but..... I kinda didn't want Buckynat in the MCU. I absolutely love comic!Buckynat. And a part of me would have really loved to have them in the MCU, and have them be MCU canon. But also, MCU!Buckynat just seem so..... bland to me. Especially Bucky. He's such an interesting character ín the comics, and they made him into a stereotypical brooding guy in the MCU. I prefer the comic buckynat dynamic so much more. Also i don't think I could have been able to bear watching the amnesia storyline on the big screen. I would have ended up bawling.
I think this is one of those things where it's all a bit connected. The reason why MCU Bucky comes off as bland is because they stripped him of basically all his stories, including buckynat. It's hard to build character personality if you're not giving him anything to do. He mostly exists as a prop to Steve's story. This also applies to Natasha. MCU Natasha borrows too much from 616 Sharon, for example. They're not letting these characters be themselves.
But anyway, I still really do think MCU Buckynat could've worked but they would've had to intentionally set it up right from the start. They should've gone for the original plan of having Sharon be the lead in The Winter Soldier, helping Steve. And Natasha could've been running her own side mission trying to track down the winter soldier, because she knows who he is. But the mcu thinks female characters are interchangeable so we got stuck with what we got. They also wrote themselves into a corner when they decided Hydra had Bucky and not the soviets, but that's a whole other problem.
I recently listened to this audio series called Black Widow: Bad Blood, and I thought that adaptation did a pretty good job at working around those plot difficulties re: all the changes the movies made to their characters, but specifically Bucky's. I recommend it if you haven't checked it out. There's an ebook version, too, but the audio version is really worth it for the immersive experience.
I don't know that any live action adaptation will ever fully satisfy, honestly, because it's so hard to compress years of stories into a movie where there's 50 other plots and characters. In some ways it's better they never tried. That means they never fucked it up.