All right--I have now had a hot shower using water from the new heater. It was just fine. So water heater now at 4 stars out of 5. Provisionally.
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oddly though, the app that will allow me to control it is "not compatible with any of [my] devices", which seems odd to me. (I've got a pixel 6a & a samsung tablet--not obscure devices).
huh--they say it's due to an app update that somehow knocked the device compat info out of whack. They say it'll be fixed in 3 days.