The tank as far as I know needs to be inside the house at least where I live so you would jave to check where you are. They have them all ver the place in Europe and I have never heard of them being high maintenance I do know if you have long plumbing runs they are not as efficient so oftem 2 are needed.

They can be located on the exterior and that typically makes them easier to vent... As for expensive maintence, mine was a lot cheaper than the repair bill after my storage water heater leaked over the interior floor...
They often need a larger gas line so check with a plumber...