I`ve probly put in hundreds over the yrs. , the biggest house I ever did , (had a bunch in it) eventually wasnt happy with them , and went back to tank type water heater.
The one lungers work for a while but all will go out in time , usually quicker than what is satiisfactory , the new gas fired tanklless , are terribly expensive , I wouldnt even consider one myself...
Is it possible that you have great amount of electralisis in your system ? Thetre is some reason you’re not getting any life out of them............??