A tankless system generally does two things, puts a restriction on the supply to ensure at least warm water and second, can vary the output radically depending on flow or inlet temperatures. You can mitigate having unreasonably hot water by installing a tempering valve at the outlet,which under ideal conditions restores the flow rate (only if the outlet water temp is hot enough).
Depending on the inlet temperatures, the flow desired, and how much of a loss you have from the point of use back to the heater, you may not have adequate outlet temperatures unless you can cascade units. Some of them make this easier than others from the limited reading I've done, but it is being addressed.
There was one of these in the house I rented in Germany nearly 30-years ago. I didn't like it then. I opted for trying one 10-years later here in the USA, it was okay, but maintenance and continued sources of parts became a major pain (recently replaced with an indirect tank).
Europeans (at least used to) have much different expectations for this type of a system and seem to live with their quirks and limitations more than we do in the land of plenty.
If your open space is not large enough, since the heat load on a tankless (let alone several in series) will mean that you won't be able to run them without introducing outside air can mean that you will end up with a large unconditioned space. The alternative is building an insulated utility room where you can install the thing so the introduction of combustion air makeup won't impact the (typically) basement. Some of them probably can be run with closed combustion with air piped in from outside which would eliminate at least the air problem. Many people ignore the requirement for makeup air for their combustion products. A gas stove, furnace, gas log fireplace, dryer,etc. all require significant quantities of air, and will be pulling that in from the cracks, gaps, and openings in the house. If they don't get enough, you can run into some real problems. DO NOT DISCOUNT THIS in your choice - newer houses tend to be much tighter than older ones. Much of the newer appliances like boilers, furnaces used closed combustion since they lend themselves to being much more efficient and to overcome some of the makeup air problem but stoves, gas fire places (or real ones) and dryers still don't operate that way.
Before you decide to go with this type of system, make sure you read and understand the temperature/flow charts AND your local water conditions.
Where I live, you would be a fool to consider installing a heater outside - the winters are just too cold.
Look at the physics of it...2.5 gallons per minute for a shower is 1200 pounds of water an hour. Say you need to raise it from 35-degrees in the winter where I live to 105, that's 70 degrees. 1200*70 = 84K BTU. Add a second shower, 168K BTU. Assume maybe 85% efficiency, and raise that to nearly 100K BTU for one shower, and 200K for two. Throw in any additional flow, and the temperature drops, and even though you have the control on max hot, it drops in temperature. Don't even think about having the washing machine or dishwasher running anywhere near the same time or your outlet temperature may drop 5-10-degrees or more.
So, you may need to make some severe compromises in lifestyle, significantly oversize the thing, and engineer impressive physical plant preparation (air, gas, flue) in order to provide the same output and then you will have more maintenance to perform. Our energy costs still do not make them more efficient at our typical expectation levels in my opinion.
Since I switched to a tank, I can fill the tub in 5-minutes where it took maybe 25 before (had to almost dribble it out to get enough temperature rise) and have much more pressure to take a shower with. Maybe if I lived in Hawaii I'd think differently, but not in NH!