I'm pretty sure the Ultegra-level Hydro brifters can come with either the new flatmount caliper or a postmount M785 caliper, so I would assume the road and mtb ones are all compatible.
if this is true I would rather run SLX calipers
If the pistons are the same diameter, there should be no problem.
@swedeee started
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I'm pretty sure the Ultegra-level Hydro brifters can come with either the new flatmount caliper or a postmount M785 caliper, so I would assume the road and mtb ones are all compatible.