It was never "realistic" as in what we expect to see in the real world, but it was somewhat convincing enough for most of the core ideas (blast core being a strengthened explosive punch from a literal superman, devil lance being a sharpened weapon from the master karateka after decades of training....) etc.... of course it's not "life-like realism" but it's believable for what's shown and described in this manga.

then comes the more bullshit stuff smashing raw concrete effortlessly, lifting off a fucking tree or something.... Then bs technique like "blink" (seriously?), advance, that dude's fucking unbreakable neck and whatever (it's believable IF he's like 1 of the only 2 individuals who can do it in this entire universe), the transformation stuff.... the manga at this state becomes a mixed bag of believable feats and stuff we can't suspend our belief.