These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Brave Frontier
Anti-Climax Boss: Most bosses can be easily beaten when you know their attack patterns.
Game Breaker: "Brave Burst spam" and "Critical hit spam" team archetypes. The former is designed as such so that all of your units can use their Brave Burst every turn. The latter is designed to make full use of Mad God Zebra's Leader Skill, which increases damage from critical hits to absurd levels, by increasing the critical rates of your units as much as possible.
Ship Tease: There's a few of this in the unit's flavor text.
Zeln/Nemia: Certain on Zeln's side, considering that his 4 stars and 5 stars evolution stated that he focused on training himself to follow her.
Vargas/Lava: Small, but there. Also crossover with Foe Yay, considering that she defeated him before opening up to others, and in her 5 stars evolution it was stated that she "wanted a rematch with Vargas even more than a battle with the gods". Also, Vargas's 6 stars evolution outright stated that the wings on his back were given to him by Lava.
Loch/Edea, at least one-sided, considering he chose to come out of his self-imposed exile to join her.
Any time you're facing Phoenix. It constantly uses its Brave Burst, which lets it regenerates for a few turns before using it again. Unless you have poison or paralysis, it'll either be a long or impossible fight, depending on whether you can damage it more than it recovers.
Miracle Totem. Not only it can cast Brave Bursts of different elements, it also has a new move, Chaos Trilogy, that can, together with using other attacks at once even knock out 5 star units!
Thunderbird and Great Falcon Ziz when summoned from the Rare Summon. This is due to their both their bad stats and Brave Bursts attacking random enemies instead of all.
And among the ones normally obtained in-game, healing units are basically the inversion. Titania and Legtos are special mentions for being easier-to-obtain (though inferior) versions of God Tree Eltri and Phoenix, respectively.
Viewer Gender Confusion: Official flavor texts explicitly refer to Atro as male, but there are many who would insist that "he" is actually a "her." The confusion was so bad, even the translator dubbed his 5-star name as "Goddess Atro" before the term was corrected in a recent version, and even then, not all of his gendered terms were entirely fixed (for example, he'll still call himself a swordswoman if obtained through Honor Summon). His somewhat androgynous appearance certainly does not help.