What is your water hardness? What fuel do you have available? Electric would require a very significant supply. So would gas. On a bigger tub, it's nice to have large flow volume so you don't wait forever to fill it. This means you must put a huge amount of energy into the water to get it hot enough as it flows by the heat exchanger (think hand through a candle - a blowtorch will get you hotter when passing by at the same rate). To get the flow for multiple showerheads and other things that may be using hot, you may need multiple units.
The varying pressure could present a problem as the temperature might either rise as the pressure drops (decreased flow), or it would be constantly adusting the burner, if it deals with that that way. There are ways to keep the pressure fairly constant with a well.