Easy to install. For the instructions the guide helps but there's also a YouTube video that is good to watch (should be included in instructions as an option as well).
One advice is I didn't know the resistance of my in-floor sensor (no longer have manual). Thankfully I have a laser thermometer gun (can get at Costco for ~20$) and was able to try a few settings before finding the one that matched the floor temperature.
Homekit setup: was fairly painless. First attempt failed (I think this is due to some quirks in HK). I just reset the Mysa and paired again. One thing is that HK thinks there's a cooling option on the thermostat (which is not correct)