If Yuuka's power is in direct correlation to her ability, then the logical thing is to say 'she can manipulate flowers in any sense of the word', because we have people like Reimu and Yukari who have powers that sound lame but are in effect extremely powerful because the wording on their ability is used literally.
But yeah, I'm a fan of the theory that her power isn't in correlation to her ability. She probably has a definite advantage in some places (like the Garden), since she gets another tool, but ultimately she's just absurdly powerful by virtue of experience and age. It's also stated "Furthermore, there were also such statements like "this ability is more of an extra," and "completely unrelated to her abilities, she uses an umbrella and scatters flower petals to perform graceful attacks" and because of this, one would think that this is not an ability oriented for battle (although one might still wonder if scattering flower petals is not included in manipulating flowers)." on the wiki. PC-98 says she's the most powerful class of youkai, but Windows doesn't say anything either way.
She picks a fight with Shikieiki pretty confidently, who is someone Yukari is scared of. (In a 'oh god she's more powerful than me' or a 'scary like my old grandma' way, who knows)
When she's beat in Phantasmagoria, it's a lot like beating Marisa in Fairy Wars. She's the one driving you away, not the other way around.
Perfect Mementon mentions she's really powerful a lot, and that even the claims that Reimu's exterminated her several times is 'probably bogus'.
Those are the sources quoted on the Touhou Wiki.