FWIW, I was looking at the Vertex, and ultimately put in a Navien because it used standard PVC pipe (I think Rannai uses special pipe or adapters), both heat exchangers are stainless steel and it has a 1/2 gallon storage tank and recirculation pump on the "A" models, which combined with and extra 1/2" pipe to any fixture in the home, gives you isntant hot water. It passed the 1 year makr and no issues. Very little debris in the strainers, but you do have to keep up on cleaning the inlet screen. The amount of crap on there makes me wonder if all HE furnaces should have a cleanable screen if the intake is anywhere near the ground outdoors. Mostly river bugs (annoying little moths... I like near a river) grass slippings and bits of leaves and debris that would clog up the burner. The overall cost was slightly less and hot water capaicty in endless wiht constant temp up to about 5GPM where I am in mid winter with 35F incomming water temps.
IF you have any water quality issues, I would only go with a storage tank unit and keep replacing the anode rod regularly. I'd also look at a storage tank like the Polaris that has a Stinless steel liner. With poor water quality, the glass lined tank like the Vertex won't last long.
Also, in terms of effcency, the Vertex is only going to be 96% efficient if you keep the water temps around 115F. Effciency will drop I suspect above that since incomming water temp determines you minimum flue gas temp and how much heat you can extract out of the flame.
The Polaris is the premium unit with a stainless steel tank and coil, PVC vent, sealed combustion, super-insulated and with burner input to 199mbtuh. Fill the big tub and recover faster than most tankless with the same combustion efficiency. We use them to heat space and not water in new construction and retrofits with radiant floors and fan coils. There are no substitutes.