There are a couple of caveats...the aqua hot need to be serviced...most people have them serviced yearly..about $100. I have mine done every other year as the nozzle wasn't bad and yearly seemed like a waste of money. That said, you can usually tell if your AH needs servicing by the smell and noticeable smoke as the nozzle becomes fouled. Ours burns with no smell or smoke so I will not service it this year. As with any mechanical item, maintenance is the key. There is a reservoir that needs to be periodically checked to ensure it's full. Similar to your coolant reservoir.
The nozzle assembly can be replaced as a DIY if you have reasonable tools and some mechanical ability. However, when I have mine serviced, they also check the pressure regulator etc. something I cannot do myself as I don't have the proper tools. IMHO, it is worth the small price to have someone knowledgeable with the proper tools to service it.
As far as whether it's a deal breaker....well, my wife would not even look at, let alone consider, buying a coach without one. I don't think I would go back to a water heater either. Long hot showers for as long as you want, and not having to wait for the WH to regenerate is worth it by itself. The fact that you can heat the coach while underway is a bonus too. The fluid is run through the engine and heated while driving so you have heat throughout the coach if you want it. The only "heater" are small individual fans that silently bring warm air into the living quarters.....yes, it would be a deal breaker for us....Dennis