Home plumbing doesn't generally do multiple pipes. I wouldn't know if that's a code thing or just a complexity thing.
But my understanding is that the heat output of the inline heaters is proportional to flow rate, so you aren't achieving much by running them in parallel instead of series.
It's not that the early devices couldn't handle 3gpm, it's that they couldn't heat 3gpm, so you got really warm water but not hot water. No idea where things stand now.
I’m really not entirely sure. I haven’t been around one since I traveled overseas and that’s been a while now. My new place has a new furnace and the old one we never got around to doing improvements so I just remember the old complaints.
Thermal conductivity seems likely. How many feet of pipe can you cram in there, especially if you have to insulate so you don’t catch the building on fire?
Or can you make a 3d counterflow system so you don’t have to insulate at all?
ETA after further reflection, I believe they generally worked better in Europe and that would most likely be down to 220 vs 120, so you may be right about power density.
Why not just run them in parallel? If you need more flow, just hook up two or three or N in parallel, feeding the same pipe?