It doesn't HAVE to be multipitch. I feel people are saying that too much lately. If you don't have the time/energy for multipitch (*raises hand*) then a well done single pitch can do just as good of a job. After all, Ritsu's Kire really just comes from that C5 pitch. If the bank was only that single pitch, a non-multipitch, it'd still sound shouty and awesome. Multipitch just adds extra layers of variation to a voice.
rant aside
Strength depends on two things I feel. #1 being a microphone of at least okay quality (If it's all muffly and picks up background noise, your tone will never get through and it'll be impossible to make a nasal enough voice). #2, the main part, is tone. All strong banks, whether just strong in their single pitch (Sorane Rana) or strong in a higher pitch (Ritsu's popular banks), that's just it - the recordings are strong. What makes a strong voice is a full, mildly nasal tone and powerful consonants that aren't airy, slow, overly crisp - they're very strong. It all depends on the recordings.
Though some people just aren't built for voicing a nasal UTAU. I have difficulty making soft UTAUs, I just naturally make strong-ish ones. I know people who have trouble doing solid, strong voices. It just depends. But other than a baseline of quality for a mic, tone is everything.