IIRC tankless water heaters seem to be most successful when used in specific locations of the house in conjunction with a "big white tank" heater for everything else.
A fancy shower or soaking tub might get its own tankless heater, or "the bathroom at the far end of the house" where it might otherwise take 5 minutes for the hot water to work its way along the pipe and out the low flow faucet.
For a family of 5 with natural gas I would go with a conventional water heater.
Disclaimer - I learned everything I know about tankless water heaters off the internet
