Depending upon the size of the heater, the typical faucet-supply hose might not allow enough flow ... but then it might be just right since you do not want excessive flow wearing out the insides of your overall plumbing anyway. Your question, however, is about whether or not the hose would hold up over time, and I would have been inclined to get a hose kit from the heater's manufacturer if I had not hard-plumbed my own heater. Whichever way you go, you should include a flow-control valve somewhere so you can have only just enough flow to keep the heat exchanger sufficiently supplied. I set the flow to my heater by watching a thermometer placed in front of it while reducing the flow until the output temp began to drop a bit, and then I opened the flow back up just enough to let the heater produce 100% of its ability.