Krasinski was fine, and I didn't mind the way she spaghettified him, because hex magic does not obey physics. The problem always is that Mr. Fantastic's powers aren't magic, he's just able to elongate and stretch himself. It's barely shown in the movie, as the only time he uses his power is to jump into the frame and reach out to try to grab Wanda after she murders Black Bolt. It is the briefest moment and yet it still doesn't look right, because his arm doesn't thin as it stretches out, and he leans forward when physics would suggest he lean back to counterbalance the shift in his center of gravity.

Every movie featuring Reed Richards has gone out of the way to avoid showing his powers.

The best example I can think of was that one ridiculous scene where Ioan Gruffud fought The Thing, and you can tell they cropped half the fight out of the frame to avoid showing more terrible CGI. Like you just see their heads bobbing around while their arms fight each other off camera, and then it awkwardly pans out to reveal Reed has Ben all tied up.