Specifically, is the XT BR-M8020 mountain caliper compatible with an Ultegra hydraulic road brifter like like the ST-R8020 or ST-R8070?
I have confirmed that they both use BH90 brake hose ("high power" rigidity). If I just ensure to use the right fittings for both sides of the hose (the banjo bolt from the SM-BH90-SBLS hose kit, and the straight brifter connectors from the SM-BH90-JK-SSR hose kit) can I be confident this is safe?
I'd heard that recent Shimano road and mountain hydraulics of the same hose (BH90 vs BH59) were mostly cross-compatible, but I just read that even inside the upcoming XTR M91xx series of mountain brakes (which both use BH90 cable), Shimano recommends a totally different mountain brake levers for the 2-piston XTR brake (the M9100) vs the XTR 4-piston model (the M9120). Why would you need a different brake lever for these different calipers with the same size hose? How could you know which post-mount caliper (the M9100 vs M9120) would work with an STI ST-R8020 brifter?
(As background, I'm looking for a good, strong post-mount hydraulic disc brake for the front of a tandem with drop bars. I will be upgrading the Shimano STI brake lever / shifter at the same time I add the hydraulic brake.)