You should spend some time reading the literature of tankless heaters - the child post below explains it. Tankess heaters can only raise temp at certain flows. So, if your incoming water is ~55 degrees, it might be able to heat to 110 degrees and flow 6.6 GPH - basically one shower. In that scenario if someone turned on the hot water for.. say... dishes, the tankless can't keep up with demand and the overall output will be colder. Probably not cold but it might not be what you wanted.

The more expensive you go, the more the tankeless can do concurrently, but the more sacrifices you'll make: they'll be physically larger, they might require a bigger gas line, etc.