I have a Cambium C15; good to go out of the box, no proofing required. I’ve used it in all sorts of weather with no problem.
Brooks have now brought out the Cambium waterproof range – C15/17/19 full saddle or carved version; see boxelder’s link^^^
Cambiums don’t need ‘breaking in’ unlike the more traditional saddles but I think a properly broken-in and regularly proofed leather Brooks will always be hard to beat. As they’ve been around for so long – and before the days of synthetic materials – I would suggest a leather Brooks is upto all conditions unless you leave it exposed to the elements for an extended period.
You could contact Oliver Parsons who is the Brooks Marketing Director – oliver@brooksengland.com – and invite him to comment.