Wallin didn't do too badly against Joshua in the first 3-4 rounds but he mentally fell apart quickly when Joshua took it up a gear, partly because Joshua had sparred him extensively in the past and handled him. He was a known quantity to Joshua in a way he wasn't to the others, particularly the overconfident Fury, which in part explains the speed in which he was beaten by Joshua. The cut also gave Wallin encouragement and operational help, and by the time he got the Joshua payday he was set economically and several years older. He likely could have survived at least a couple more rounds if he'd felt the need to continue.
His wins on paper I'd rate higher than Wallin, especially the method. Is he overhyped by some? Sure, but name a heavyweight who didn't start overhyped.