Two things.
First, I couldn't agree more with everyone who finds the redesign of the Health app a huge step backward. I totally agree.
Second, putting possible security issues aside (I'm just speculating about that), visualizing things on a phone is certainly very limited, and like you I'd find it much easier on an iPad. Even with the larger number of pixels on the Xr/11 and Max phones, it'd be far easier to take in things like trend information, which I think is a big deal for health information, on an iPad.