Judging by Oriental Sacred Place, most fairies run like hell while screaming like little girls whenever they feel threatened. Presumably their attacks on Reimu and Marisa in the games are mostly them trying to play pranks (although most extra stages typically have the story or dialogue describe the fairies as "going wild" etc etc). After all, also judging by Oriental Sacred Place, the attitude of most fairies seems to be "prank first, worry about punishment later" (though they at least have enough self-preservation to try to not get caught while pranking).
Perfect Memento explicitly states that Yuuka's really REALLY strong, and explicitly states that Yuuka's strength has nothing to do with her ability to control flowers (and, by logic, vice-versa).
I always took the whole "force of nature" sentence to basically mean that Yuuka will attack without discussion or reason. In essence, trying to talk your way out of a battle with Yuuka is like trying to talk your way out of battle with an incoming tornado. Although apparently you're allowed to watch from the sidelines if she's in a battle with someone else.
Although Perfect Memento describes it very seriously, from Yuuka's perspective, she's really just playing around and teasing people according to Phantasmagoria of Flower View. Fandom took her whole genocide comments and remarks from the pre-windows games and assumed she was serious about that. But ZUN explicitly stated in the now-defunct Gensokyo bulletin board that comments such as that were akin to pro-wrestling comments (IE, meant to make things look all flashy and serious and deadly when it really... well, isn't). Really, nearly ALL Touhou characters talk in very serious tones but obviously aren't actually killing each other (Remilia calilng Reimu a serial killer for killing Sakuya in EoSD, for example, despite how Sakuya's obviously still alive, and the myriad of youkai bosses saying they're going to eat you when in truth, everyone knows they're not actually allowed to eat Gensokyo residents)
...despite that, Shikieiki still thinks Yuuka takes things too far, but whatever.
Come to think about it, as to WHY she's like that... well, hilariously enough, one of the endings to Yuuka's debut game Lotus Land Story has
Given Momiji's background about old youkai running out of ways to pass the time, I'm going to guess that Yuuka's the way she is simply because she's lived so long that she has nothing else to do. Maybe that's why Shikeiki tells her she's lived too long.
Back on topic, presumably most of Fairy War's difficulty can be chalked up to being from Cirno's perspective. Marisa even explicitly states before and after the battle that she's holding back a lot, despite her being an EX boss in that game (and a very difficult one, at that!). Although she does at least akcnowledge that Cirno turned out to be stronger than she thought CIrno would be.
