Mine uses 3 x 40A for the electrical, if I open everything I got to max it out, it will go to 104A, which is the de-rate you mention. Typically a shower for me it will pull about 60A. So with a 1 shower house, plenty of margin. I have a 200A service, no issues. Here it's a matter of where you live. I grew up in country, so all houses are 200 Amps. Moving to an urban area I was shocked to find 100A to be the norm, but looks like that was largely due to Natural Gas service being around, and code hasn't yet banned anything less than 200A. So yes, people will be limited. But of course trying to run a big gas tankless unit will be a struggle for a lot of folks too. Upgrade to a 200A service is just a good all around idea, was one of the first things I did to my house, and certainly cheaper/easier than getting a gas service upgraded (if a person can get gas)
Far as getting water heated up, I experience no issues. Yes, it takes a few moments. But so does pushing cold water out of the pipes with a tanked unit. I do not see any difference in getting hot out of the facet verses when I had the NG tanked unit.
The biggest issue I see facing tankless units is flawed building code/water conservation code. As some things are power based (GPM) and other things are energy based (gallons). Sizing a tankless becomes difficult because code hasn't forced ever water using item to be "power based" with GPM limits so you can size to a worst case flow needs. Washers being the main issue. Modern ones pulse water flow, and don't have a limit on the flow. I put a flow meter inline to see what my front loader would do, it was in the 4GPM range for a couple seconds at a time. They need to get them limited to the same low flow rules as showers. Get devices that our based on Gallons to have flow limits too, and in the case of washers, store some of the water so it becomes a continuous flow, and you will be able to size tankless for houses much easier. In time if houses develop any form of power management, it will be even easier. When devices can talk and do things like command the AC to turn off while the Water heater runs, give priority to Ovens, have EV cars charge only when other items aren't using the power. When that happens then tankless will meet needs without concern of kicking a breaker.