Rinnai is a good brand I have installed several and never had a problem just make make sure you have adequate gas supply pressure, I have never seen this as a problem but it is really the only variable that would prevent it from working correctly as long as you purchased the correct model for your water demand
As for the benefits you are able to set the temp on outlet and it is accurate within 2 degrees. This means you can turn on only the hot water in the shower not have to worry about scalding and never run out of hot water as for yearly cost they are more efficient because they arent trying to keep 50 gallons of water preheated constantly the heater just kicks on when you open a hot side faucet
Although you are correct about initial cost this particular brand has its own venting system (mount it on an outside wall and it shouldnt be to costly.