Compared to a neutral-element team with -70% ATK (admittedly somewhat of a temporary exploit), an element-advantaged team with -20% ATK would be taking double damage.
BakaHentai has excellent kamihime in just about every team. Given multiple excellent SSRs, basic knowledge of mechanics, and unlimited free time, I suspect players could achieve success with just about any half-decent build strategy. I've also seen a savvy player (who didn't spend on SSR kamihime tickets) use the same build strategy as BakaHentai and fail to clear content, which is one reason I'm skeptical. You can't go wrong by building around a strong mono-element team, but you can certainly dig a hole for yourself by relying on elemental advantage to clear content, investing in all elements equally, and then belatedly realizing that some of your teams aren't up to snuff. And from the perspective of someone who writes guides for the widest audience possible, it's easier to provide optimal build templates to aim for than to individually explain to each new player how to work with the hodgepodge of kamihime they've been given in each element for each encounter.