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I am planning to run a cable actuated hydro disk brake on a CAAD13 frame, but my bike mechanic said that it might not be possible because of how the rear brake hose runs below the bottom bracket on CAAD13.
Is his assessment correct, or would I be able to do this and be satisfied with the braking?
I haven't done it, but would also be hesitant that the cable setup might be irreconcilably bad. This picture from the CAAD13 supplemental manual shows the hose routing at the BB:
And this one shows how it exits the NDS chainstay:
The housing in the BB area will have some angular bends forced into it. That is a scenario where cable friction issues can be a real problem.
From a mechanic perspective in a situation like this, it's impossible to say how it's going to turn out until you do it, and these are time-consuming bikes to set up the internal routing on even when you're not experimenting. Compounding the issue is that universal statements cannot be made about setup quality with all mechanical flatmount brakes you might choose here, because the housing entry angle an individual caliper gives you can have a dramatic effect on the friction level. Tight bends between the housing stop on the caliper and the exit point on the frame can be a major stumbling block when installing a mechanical brake on a frame designed with hydros in mind, or on flatmount mechanical bikes in general.
If you're willing to modify the frame/routing hardware such that the housing shoots straight out the bottom of the BB and then loops back up toward the caliper, zip-tying it to the frame where it passes by, that can often solve all these problems. (See red line I drew in the image). Not every mechanic is going to see it as their job to redesign the routing and drill the necessary holes etc though. (I believe for CAAD13 it would only mean putting a hole in the access panel cover, which is replaceable). And, that's to say nothing of negotiating with a customer about altering the look away from the clean internal routed aesthetic that they presumably bought the bike for in the first place.
Note that the whole deal with how the routing on this bike works is the hose is below the "Guide Body" and the gear cables run on top of it. The order of operations is finicky at the best of times, and doing anything with the brake line once in place means dealing with the gear cables.
