If you are talking about those units that look like tankless water heaters, do your research. Anything above 200,000 BTU is truly considered a "boiler"; anything less is a heater regardless of what they call it. If it's truly a "boiler" (>200,000 BTU) you need a specialized plumber with a boiler license.
I point this out since you have to determine what your heat load will be for the home, as well as what your hot water demands will be. Some families will require enough simultaneous hot water usage such as a 199,900 BTU tankless alone will be needed to reach the desired temp rise and flow rate. If that's the case, there's no room for heat--unless you get a second unit. If you have a second unit, well--why bother with a combo?
My older house (built 1992) had a furnace of 125,000 BTU and a 50 gal water heater that was probably 40,000 BTU. However, there were similar sized homes in my neighborhood that had either a 75 gallon or two 50s for the water to meet family demand...
So, do your calculations before making a decision.
A few things to consider. Combi boilers are designed to do two different things with different needs, so they will not be as efficient as a stand-alone boiler. Combi boilers can only supply a limited number of GPM at a given temperature rise (which varies with climate zone). Some use a plate heat exchanger and some use a small tank. Combi boilers are usually condensing boilers. They get their extra efficiency by condensing the water vapor in the flue gases (the latent heat of vaporization). If your heat distribution is with baseboard convectors or radiators, the water back to the boiler won't be cool enough to condense that water vapor, so you won't get the published efficiency. Combi boilers, and condensing boilers and tankless water heaters, are more expensive to install and more expensive to maintain. If they are not serviced regularly, they will be more expensive to repair. One advantage of condensing boilers is that they can be vented with PVC out a sidewall.
You might want to consider a boiler separate from the water heater. Consider a heat pump water heater, if it will meet your needs.