Can a Shimano XT (mountain) hydraulic disc brake caliper be used with an Ultegra STI road brake lever? 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.)
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? 


The key is that there are two brake options available: "race" and "trail". "Race" features 2-pot calipers and lever without Servo Wave™, which yields different modulation curve (more linear?). The "trail" version is 4-pot and with Servo Wave™ levers, which has more of an on-off modulation.

As far as I know, all Shimano calipers/levers have very close displacement/pressure values to be deemed compatible. On one hand, I did not hear about incompatibilities between any pre-9100 parts, and on the the other there's a lot of reports of mix-matching working with great success. Unless there are reports stating otherwise, I'd bet on compatibility trend. In the end, calipers are not that expensive to experiment with and there are cheaper options (BR-M520, BR-M640).


  How could you know which post-mount caliper (the M9100 vs M9120) would work with an STI ST-R8020 brifter?


What Shimano "recommends" and what actually works are not necessary same things. Shimano is a conservative company bound by legal responsibilities. If you want 4-piston calipers to pair with STIs, take a look at Hope RX-4 SH (explicitly compatible, I run these) or Shimano calipers mentioned above.

As with all things new, there's not much feedback, you might as well get that caliper and report results here as an answer.