I disagree completely. We are talking about shipping chemistry, not chemistry in the show where none of the yuri pairings has any. Or am I supposed to believe Ruby, the annoying, hyperactive kid with zero interest in romance, and Weiss, the aspiring to be perfect heiress into guys, who tolerates her is an example of good chemistry? No their romantic potential comes from ignoring those traits and viewing them less as Ruby and Weiss and more as their core personality traits of being hyperactive and tsun.
Petals has very good chemistry. All it would take is Ruby being captured or Cinder being captured for them to immediately be in an extremely promising situation, not that I'm saying RT would ever do that.
You put either of them in that compromised situation where they have time, and the thing that is going immediately going to happen is Ruby asking for a reason.
If you can't see the overwhelming chemistry in this opposition; a naive child who's lived a sheltered life filled with dreams of a black and white world where everyone is happy except when evil stirkes and the grimm are just monsters and she's going to grow up to be a hero who saves people and makes the world better--and a lost girl who (it's pretty heavily implied) suffered when young and saw the world for what it really was, cursing it and deciding to embrace the darkness and become stronger even if it means leaving the world in ashes...
...or see the spark that could form there as innocent Ruby who can't help but believe in fairy tales starts to think she might be able to save Cinder (or starts to be overwhelmed by a world she's never seen before) and Cinder, who's lived her whole life in shadow is now faced with this incredibly bright spark she can't decide if she wants to embrace out of some faint echo of hope that she'd be saved when she was young, or snuff out the light and drag Ruby down to her level... well, I can't help you.